Investments - Carrying Amount of Equity Method Limited Partnership Interests (Details) - USD ($) $ in Millions |
Sep. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Net Investment Income [Line Items] | ||
| Limited partnership interests | $ 1,093.5 | $ 1,100.6 |
| Commercial mortgage loan funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 523.6 | 590.7 |
| Private equity funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 175.4 | 118.3 |
| Real estate equity funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 108.2 | 127.5 |
| Infrastructure debt funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 79.4 | 73.9 |
| Venture capital funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 75.4 | 50.0 |
| Infrastructure equity funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | 46.8 | 52.4 |
| Other funds | ||
| Net Investment Income [Line Items] | ||
| Limited partnership interests | $ 84.7 | $ 87.8 |