Investments - Carrying Amount of Equity Method Limited Partnership Interests (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 1,137.8	$ 1,121.3
Commercial mortgage loan funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	593.2	596.0
Real estate equity funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	146.5	144.7
Private equity funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	112.5	102.2
Infrastructure equity funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	83.6	81.7
Infrastructure debt funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	66.5	69.0
Other funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 135.5	$ 127.7