Investments - Carrying Amount of Equity Method Limited Partnership Interests (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Net Investment Income [Line Items]
Limited partnership interests	$ 1,096.4	$ 1,100.6
Commercial mortgage loan funds
Net Investment Income [Line Items]
Limited partnership interests	585.5	590.7
Real estate equity funds
Net Investment Income [Line Items]
Limited partnership interests	123.8	127.5
Private equity funds
Net Investment Income [Line Items]
Limited partnership interests	122.8	118.3
Infrastructure equity funds
Net Investment Income [Line Items]
Limited partnership interests	51.1	52.4
Infrastructure debt funds
Net Investment Income [Line Items]
Limited partnership interests	72.9	73.9
Other funds
Net Investment Income [Line Items]
Limited partnership interests	$ 140.3	$ 137.8