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Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 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
Fixed maturities, trading
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		0.3
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	$ 352.9	388.4	86.0
Transfers out of Level 3 into Level 2	460.8	654.7	78.6
Available-for-sale | States and political subdivisions
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	82.4
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	175.8	342.0	4.1
Transfers out of Level 3 into Level 2		255.2
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 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	74.1		69.9
Transfers out of Level 3 into Level 2	435.8	330.6
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	20.6	46.1	8.3
Transfers out of Level 3 into Level 2	$ 25.0	$ 68.9	$ 76.3

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