Investments - Carrying Amount of Equity Method Limited Partnership Interests (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 505.2	$ 449.0
Senior commercial mortgage loan funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	197.3	149.6
Hedge funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	62.6	63.2
Infrastructure debt funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	58.4	58.3
Infrastructure equity funds
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	56.2	52.1
Other
Schedule of Equity Method Investments [Line Items]
Limited partnership interests	$ 130.7	$ 125.8