Investments - Carrying Amount of Equity Method Limited Partnership Interests (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 585.7	$ 449.0
Senior commercial mortgage loan funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	254.8	149.6
Hedge funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	50.7	63.2
Infrastructure debt funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	58.1	58.3
Infrastructure equity funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	57.3	52.1
Other
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 164.8	$ 125.8