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Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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 2 into Level 3			$ 2.3
Transfers out of Level 3 into Level 1			0.2
Transfers out of Level 3 into Level 2		173.2	40.9
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	388.4	86.0	59.3
Transfers out of Level 3 into Level 2	654.7	78.6	385.6
Available-for-sale | Non-U.S. governments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2			1.3
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	342.0	4.1	1.0
Transfers out of Level 3 into Level 2	255.2		54.8
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	0.3	3.7	3.6
Transfers out of Level 3 into Level 2		2.3	1.5
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		69.9	54.7
Transfers out of Level 3 into Level 2	330.6		237.0
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	46.1	8.3
Transfers out of Level 3 into Level 2	68.9	$ 76.3	91.0
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	$ 0.3		$ 3.7

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