Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments | ||
| Net unrealized gains (losses) on fixed maturities, available-for-sale | $ (4,564.3) | $ (4,081.8) |
| Net unrealized gains (losses) on derivative instruments | 1.4 | (40.0) |
| Adjustments for assumed changes in amortization patterns | 4.7 | 4.2 |
| Adjustments for assumed changes in policyholder liabilities | 31.1 | 21.2 |
| Net unrealized gains (losses) on other investments and noncontrolling interest adjustments | (80.1) | (58.3) |
| Provision for deferred income tax benefits (taxes) | 996.4 | 893.7 |
| Net unrealized gains (losses) on available-for-sale securities and derivative instruments | $ (3,610.8) | $ (3,261.0) |