diff --git a/src/components/balance/BalanceAccountsTable.tsx b/src/components/balance/BalanceAccountsTable.tsx
index ef93e42..c36585a 100644
--- a/src/components/balance/BalanceAccountsTable.tsx
+++ b/src/components/balance/BalanceAccountsTable.tsx
@@ -28,6 +28,7 @@ import {
   AlertTriangle,
   ChevronDown,
   ChevronRight,
+  ListTree,
 } from "lucide-react";
 import type {
   AccountLatestSnapshot,
@@ -86,6 +87,12 @@ interface BalanceAccountsTableProps {
   periodAnchor: AccountPeriodAnchor[];
   onArchiveAccount?: (account: AccountLatestSnapshot) => void;
   onLinkTransfers?: (account: AccountLatestSnapshot) => void;
+  /**
+   * Open the "détailler en titres" wizard for a *simple* account (Issue #215).
+   * The action is only offered on `kind === 'simple'` rows; once detailed, the
+   * flip is one-way (service backstop #212) so no inverse action is exposed.
+   */
+  onDetailAccount?: (account: AccountLatestSnapshot) => void;
   /**
    * Earliest snapshot date across the whole profile, used to anchor the
    * "depuis création" horizon. Falls back to "1A" range if not provided
@@ -116,6 +123,7 @@ export default function BalanceAccountsTable({
   periodAnchor,
   onArchiveAccount,
   onLinkTransfers,
+  onDetailAccount,
   sinceCreationDate,
   latentGainByAccount = {},
   latentGainRollup,
@@ -555,14 +563,19 @@ export default function BalanceAccountsTable({
                   </button>
                   {openMenuFor === acc.account_id && (
                     <div className="absolute right-2 top-full z-10 mt-1 bg-[var(--card)] border border-[var(--border)] rounded-lg shadow-md py-1 min-w-[180px] text-left">
-                      <button
-                        type="button"
-                        disabled
-                        className="block w-full px-3 py-2 text-sm text-[var(--muted-foreground)] cursor-not-allowed"
-                        title={t("balance.overview.detailComingSoon")}
-                      >
-                        {t("balance.overview.detailAction")}
-                      </button>
+                      {onDetailAccount && acc.kind === "simple" && (
+                        <button
+                          type="button"
+                          onClick={() => {
+                            setOpenMenuFor(null);
+                            onDetailAccount(acc);
+                          }}
+                          className="flex items-center gap-2 w-full px-3 py-2 text-sm hover:bg-[var(--muted)]/40"
+                        >
+                          <ListTree size={14} />
+                          {t("balance.detailWizard.action")}
+                        </button>
+                      )}
                       {onLinkTransfers && (
                         <button
                           type="button"
diff --git a/src/components/balance/DetailAccountWizard.test.ts b/src/components/balance/DetailAccountWizard.test.ts
new file mode 100644
index 0000000..50a4aac
--- /dev/null
+++ b/src/components/balance/DetailAccountWizard.test.ts
@@ -0,0 +1,35 @@
+// DetailAccountWizard — unit tests (Issue #215).
+//
+// NOTE: this project has no jsdom / @testing-library harness (see
+// SecurityPicker.test.ts, StarterAccountsModal.test.tsx). We exercise the pure
+// toggle-payload builder — the load-bearing decision of the wizard — directly.
+// DOM rendering / the confirm click are not exercised here; the wizard is thin
+// orchestration over `buildDetailToggleInput` + `updateBalanceAccount`.
+
+import { describe, it, expect } from "vitest";
+import { buildDetailToggleInput } from "./DetailAccountWizard";
+
+describe("buildDetailToggleInput", () => {
+  it("flips kind to detailed and pins the pivot to today's local civil day", () => {
+    // 2026-06-06 in the local civil day (midday avoids any DST edge confusion).
+    const today = new Date(2026, 5, 6, 12, 30, 0);
+    const input = buildDetailToggleInput(today);
+    expect(input).toEqual({ kind: "detailed", detailed_since: "2026-06-06" });
+  });
+
+  it("zero-pads single-digit month and day to a YYYY-MM-DD snapshot shape", () => {
+    // January 2nd → "2026-01-02", matching normalizeSnapshotDate's ISO regex.
+    const today = new Date(2026, 0, 2, 9, 0, 0);
+    const input = buildDetailToggleInput(today);
+    expect(input.detailed_since).toBe("2026-01-02");
+    expect(input.kind).toBe("detailed");
+  });
+
+  it("never emits a detailed → simple downgrade or an envelope mutation", () => {
+    const input = buildDetailToggleInput(new Date(2026, 11, 31, 23, 0, 0));
+    // Only the two pivot fields are set; vehicle_type/name/etc. stay untouched
+    // so updateBalanceAccount's read-and-rewrite preserves them.
+    expect(Object.keys(input).sort()).toEqual(["detailed_since", "kind"]);
+    expect(input.kind).not.toBe("simple");
+  });
+});
diff --git a/src/components/balance/DetailAccountWizard.tsx b/src/components/balance/DetailAccountWizard.tsx
new file mode 100644
index 0000000..22fc698
--- /dev/null
+++ b/src/components/balance/DetailAccountWizard.tsx
@@ -0,0 +1,163 @@
+// DetailAccountWizard — light confirmation modal that flips a *simple* balance
+// account to *detailed* entry mode. Issue #215 (Bilan détail par titres #5).
+//
+// Decision 2026-06-04 (plan-overnight): TOGGLE-ONLY. The wizard does NOT capture
+// any titles. It sets `kind='detailed'` and `detailed_since = today` (the
+// authoritative pivot date) via `updateBalanceAccount`. Per-security holdings
+// are entered at the NEXT normal snapshot — `validateDetailedSnapshot` tolerates
+// pre-pivot aggregated lines and requires holdings only at/after the pivot.
+//
+// Irreversibility: once holdings exist at/after the pivot, the service backstop
+// (#212) rejects a `detailed → simple` downgrade with
+// `account_kind_detailed_has_holdings`. The UI exposes no "simplify" action, so
+// this confirmation makes the one-way nature explicit before committing.
+//
+// Mirrors the LinkTransfersModal idiom (createPortal overlay, stopPropagation
+// inner card, i18n-only copy, WebKitGTK-safe close button).
+
+import { useState } from "react";
+import { createPortal } from "react-dom";
+import { useTranslation } from "react-i18next";
+import { X, Loader2, AlertTriangle } from "lucide-react";
+import {
+  updateBalanceAccount,
+  BalanceServiceError,
+} from "../../services/balance.service";
+import type { UpdateBalanceAccountInput } from "../../services/balance.service";
+
+/** Local civil-day ISO (YYYY-MM-DD) — matches stored snapshot date format. */
+function localISO(d: Date): string {
+  const yy = d.getFullYear();
+  const mm = String(d.getMonth() + 1).padStart(2, "0");
+  const dd = String(d.getDate()).padStart(2, "0");
+  return `${yy}-${mm}-${dd}`;
+}
+
+/**
+ * Pure builder for the toggle payload — exported for unit testing. Produces the
+ * exact `updateBalanceAccount` input the wizard commits: flip to detailed and
+ * pin the pivot to `today`'s local civil day. `normalizeSnapshotDate` on the
+ * service side accepts this YYYY-MM-DD shape verbatim.
+ */
+export function buildDetailToggleInput(today: Date): UpdateBalanceAccountInput {
+  return { kind: "detailed", detailed_since: localISO(today) };
+}
+
+export interface DetailAccountWizardProps {
+  accountId: number;
+  accountName: string;
+  onClose: () => void;
+  /** Fired after the account was successfully flipped to detailed. */
+  onDetailed?: () => void;
+}
+
+export default function DetailAccountWizard({
+  accountId,
+  accountName,
+  onClose,
+  onDetailed,
+}: DetailAccountWizardProps) {
+  const { t } = useTranslation();
+  const [submitting, setSubmitting] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  // Pivot resolved once at render: the local civil day at confirmation time.
+  const pivot = localISO(new Date());
+
+  async function handleConfirm() {
+    setSubmitting(true);
+    setError(null);
+    try {
+      await updateBalanceAccount(accountId, buildDetailToggleInput(new Date()));
+      onDetailed?.();
+      onClose();
+    } catch (e) {
+      if (e instanceof BalanceServiceError) {
+        setError(
+          t(`balance.detailWizard.errors.${e.code}`, {
+            defaultValue: e.message,
+          })
+        );
+      } else {
+        setError(e instanceof Error ? e.message : String(e));
+      }
+      setSubmitting(false);
+    }
+  }
+
+  return createPortal(
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50"
+      onClick={onClose}
+    >
+      <div
+        className="bg-[var(--card)] border border-[var(--border)] rounded-xl shadow-xl w-full max-w-lg flex flex-col"
+        onClick={(e) => e.stopPropagation()}
+      >
+        <div className="flex items-center justify-between px-5 py-4 border-b border-[var(--border)]">
+          <h2 className="text-lg font-semibold">
+            {t("balance.detailWizard.title", { account: accountName })}
+          </h2>
+          <button
+            type="button"
+            onClick={onClose}
+            className="p-1 rounded hover:bg-[var(--muted)]/40"
+            aria-label={t("common.close")}
+          >
+            <X size={18} />
+          </button>
+        </div>
+
+        <div className="px-5 py-4 space-y-3 text-sm">
+          <p>{t("balance.detailWizard.intro")}</p>
+
+          <ul className="list-disc pl-5 space-y-1.5 text-[var(--muted-foreground)]">
+            <li>{t("balance.detailWizard.pointFrozen")}</li>
+            <li>{t("balance.detailWizard.pointNextSnapshot")}</li>
+            <li>{t("balance.detailWizard.pointPivot", { date: pivot })}</li>
+          </ul>
+
+          <div className="flex items-start gap-2 rounded-lg border border-amber-500/30 bg-amber-500/10 px-3 py-2 text-[var(--foreground)]">
+            <AlertTriangle
+              size={16}
+              className="mt-0.5 shrink-0 text-amber-500"
+              aria-hidden
+            />
+            <span>{t("balance.detailWizard.irreversible")}</span>
+          </div>
+
+          {error && (
+            <div className="text-xs text-[var(--negative)]">{error}</div>
+          )}
+        </div>
+
+        <div className="px-5 py-3 border-t border-[var(--border)] flex items-center justify-end gap-2">
+          <button
+            type="button"
+            onClick={onClose}
+            disabled={submitting}
+            className="px-3 py-1.5 text-sm rounded border border-[var(--border)] hover:bg-[var(--muted)]/30 disabled:opacity-50"
+          >
+            {t("common.cancel")}
+          </button>
+          <button
+            type="button"
+            onClick={handleConfirm}
+            disabled={submitting}
+            className="px-3 py-1.5 text-sm rounded bg-[var(--primary)] text-white disabled:opacity-50 disabled:cursor-not-allowed"
+          >
+            {submitting ? (
+              <span className="flex items-center gap-1.5">
+                <Loader2 className="animate-spin" size={14} />
+                {t("balance.detailWizard.confirming")}
+              </span>
+            ) : (
+              t("balance.detailWizard.confirm")
+            )}
+          </button>
+        </div>
+      </div>
+    </div>,
+    document.body
+  );
+}
diff --git a/src/i18n/locales/en.json b/src/i18n/locales/en.json
index 7a4976a..eb01702 100644
--- a/src/i18n/locales/en.json
+++ b/src/i18n/locales/en.json
@@ -1550,9 +1550,7 @@
       "latestValue": "Latest value",
       "periodDelta": "Δ% over period",
       "noAccounts": "No active accounts. Create a balance account to get started.",
-      "accountsTitle": "Accounts",
-      "detailAction": "Details",
-      "detailComingSoon": "Available in a future release."
+      "accountsTitle": "Accounts"
     },
     "period": {
       "legend": "Analysis period",
@@ -1861,6 +1859,20 @@
     "vehicle": {
       "none": "No envelope"
     },
+    "detailWizard": {
+      "action": "Detail into securities",
+      "title": "Detail “{{account}}” into securities",
+      "intro": "This account will switch to per-security detailed entry. Securities are entered at the next regular snapshot — this wizard captures none right now.",
+      "pointFrozen": "Past aggregated history stays frozen read-only: older snapshots keep their single total value.",
+      "pointNextSnapshot": "From today on, every new snapshot will require the per-security breakdown (with cost basis).",
+      "pointPivot": "Pivot date: {{date}}.",
+      "irreversible": "One-way action: once securities are entered, this account can no longer return to aggregated entry.",
+      "confirm": "Detail",
+      "confirming": "Switching…",
+      "errors": {
+        "account_kind_detailed_has_holdings": "This account already has securities entered and can no longer return to aggregated entry."
+      }
+    },
     "transfers": {
       "linkAction": "Link transfers",
       "direction": {
diff --git a/src/i18n/locales/fr.json b/src/i18n/locales/fr.json
index 07a9860..0727c9f 100644
--- a/src/i18n/locales/fr.json
+++ b/src/i18n/locales/fr.json
@@ -1550,9 +1550,7 @@
       "latestValue": "Dernière valeur",
       "periodDelta": "Δ% sur la période",
       "noAccounts": "Aucun compte actif. Commencez par créer un compte de bilan.",
-      "accountsTitle": "Comptes",
-      "detailAction": "Détail",
-      "detailComingSoon": "Disponible dans une prochaine version."
+      "accountsTitle": "Comptes"
     },
     "period": {
       "legend": "Période d'analyse",
@@ -1861,6 +1859,20 @@
     "vehicle": {
       "none": "Sans enveloppe"
     },
+    "detailWizard": {
+      "action": "Détailler en titres",
+      "title": "Détailler « {{account}} » en titres",
+      "intro": "Ce compte passera en saisie détaillée par titre. Les titres se saisissent au prochain instantané (snapshot) normal — cet assistant ne capture aucun titre maintenant.",
+      "pointFrozen": "L'historique agrégé passé reste figé en lecture seule : les anciens instantanés conservent leur valeur globale.",
+      "pointNextSnapshot": "À partir d'aujourd'hui, chaque nouvel instantané exigera le détail par titre (avec coût d'acquisition).",
+      "pointPivot": "Date de bascule (pivot) : {{date}}.",
+      "irreversible": "Action à sens unique : une fois des titres saisis, ce compte ne pourra plus revenir en saisie agrégée.",
+      "confirm": "Détailler",
+      "confirming": "Bascule…",
+      "errors": {
+        "account_kind_detailed_has_holdings": "Ce compte a déjà des titres saisis et ne peut plus revenir en saisie agrégée."
+      }
+    },
     "transfers": {
       "linkAction": "Lier transferts",
       "direction": {
diff --git a/src/pages/BalancePage.tsx b/src/pages/BalancePage.tsx
index 838d366..f17565a 100644
--- a/src/pages/BalancePage.tsx
+++ b/src/pages/BalancePage.tsx
@@ -34,6 +34,7 @@ import BalanceOnboardingCard from "../components/balance/BalanceOnboardingCard";
 import BalanceEvolutionChart from "../components/balance/BalanceEvolutionChart";
 import BalanceAccountsTable from "../components/balance/BalanceAccountsTable";
 import LinkTransfersModal from "../components/balance/LinkTransfersModal";
+import DetailAccountWizard from "../components/balance/DetailAccountWizard";
 import StarterAccountsModal from "../components/balance/StarterAccountsModal";
 import { getPreference, setPreference } from "../services/userPreferenceService";
 import { renderCategoryLabelFromAccount } from "../utils/renderCategoryLabel";
@@ -52,6 +53,10 @@ export default function BalancePage() {
   const [linkTarget, setLinkTarget] = useState<AccountLatestSnapshot | null>(
     null
   );
+  // Issue #215 — "détailler en titres" wizard target (a simple account being
+  // flipped to detailed entry mode).
+  const [detailTarget, setDetailTarget] =
+    useState<AccountLatestSnapshot | null>(null);
   const [categories, setCategories] = useState<Category[]>([]);
   const [transfersByAccount, setTransfersByAccount] = useState<
     Map<number, BalanceAccountTransferWithTransaction[]>
@@ -324,6 +329,7 @@ export default function BalancePage() {
                 vehicleLabels={vehicleLabels}
                 onArchiveAccount={(acc) => handleArchiveAccount(acc.account_id)}
                 onLinkTransfers={(acc) => setLinkTarget(acc)}
+                onDetailAccount={(acc) => setDetailTarget(acc)}
               />
             </div>
           </div>
@@ -348,6 +354,17 @@ export default function BalancePage() {
           }}
         />
       )}
+
+      {detailTarget && (
+        <DetailAccountWizard
+          accountId={detailTarget.account_id}
+          accountName={detailTarget.account_name}
+          onClose={() => setDetailTarget(null)}
+          onDetailed={() => {
+            void reload();
+          }}
+        />
+      )}
     </div>
   );
 }