Fair Value Accounting - Change in Level 3 Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||||
| Changes included in OCI, net of tax | $ (0.9) | $ (4.1) | $ 0.1 | $ (3.0) |
| Junior Subordinated Debt | ||||
| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||||
| Change in fair value | (1.2) | (5.4) | 0.1 | (3.9) |
| Junior Subordinated Debt | Fair Value, Recurring | ||||
| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||||
| Beginning balance | (69.9) | (63.2) | (71.2) | (64.7) |
| Ending balance | $ (71.1) | $ (68.6) | $ (71.1) | $ (68.6) |