Fair Value Measurements - Transfers (Details) - Available-for-sale - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 119.5	$ 228.7
Transfers out of Level 3 into Level 2	111.6	58.1
Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	39.5	10.4
Transfers out of Level 3 into Level 2		42.9
Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		15.2
Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	80.0	$ 218.3
Transfers out of Level 3 into Level 2	$ 111.6