Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2, net derivative assets (liabilities)			$ 26.5
Other investments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				$ 10.0
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		$ 172.5		173.2
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 217.3	33.7	388.1	33.7
Transfers out of Level 3 into Level 2	289.5	2.3	525.6	28.3
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	217.3		342.0
Transfers out of Level 3 into Level 2	102.6		148.9
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		3.7		3.7
Transfers out of Level 3 into Level 2		2.3		2.3
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		$ 30.0		30.0
Transfers out of Level 3 into Level 2	167.3		314.6
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			46.1
Transfers out of Level 3 into Level 2	19.6		62.1	$ 26.0
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	$ 0.3		$ 0.3