Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Derivative assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	$ 0.3		$ 0.3
Available-for-sale \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	13.1	$ 116.6	188.3	$ 247.2
Transfers out of Level 3 into Level 2	65.5	225.4	169.5	225.4
Available-for-sale \| States and political subdivisions
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	12.0		12.0
Available-for-sale \| Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	1.1	80.3	176.3	142.5
Available-for-sale \| Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	10.2		10.2
Available-for-sale \| Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		36.3		84.1
Transfers out of Level 3 into Level 2	$ 55.3	$ 225.4	139.3	225.4
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			20.0
Trading \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			$ 29.0