Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Derivative assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2, net derivative assets (liabilities)			$ 0.3
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		$ 25.1	188.3	$ 272.3
Transfers out of Level 3 into Level 2	$ 11.2	197.8	180.7	423.2
Available-for-sale | States and political subdivisions
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			12.0
Transfers out of Level 3 into Level 2	10.2		10.2
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		25.1	176.3	167.6
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2			10.2
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				84.1
Transfers out of Level 3 into Level 2		172.8	139.3	398.2
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				20.6
Transfers out of Level 3 into Level 2	$ 1.0	$ 25.0	21.0	$ 25.0
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			$ 29.0