Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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				173.2
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 0.3		388.4	33.7
Transfers out of Level 3 into Level 2	31.3	$ 7.3	556.9	35.6
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			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	3.7
Transfers out of Level 3 into Level 2				2.3
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				30.0
Transfers out of Level 3 into Level 2			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	$ 6.8	$ 7.3	68.9	$ 33.3
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2			$ 0.3