Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2, net derivative assets (liabilities)				$ 26.5
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 25.1	$ 0.3	$ 272.3	388.4
Transfers out of Level 3 into Level 2	197.8	31.3	423.2	556.9
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	25.1		167.6	342.0
Transfers out of Level 3 into Level 2		24.5		173.4
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		0.3		0.3
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		398.2	314.6
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			20.6	46.1
Transfers out of Level 3 into Level 2	$ 25.0	$ 6.8	$ 25.0	68.9
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2				$ 0.3