Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 232.3	$ 51.1	$ 42.8
Transfers out of Level 3 into Level 2	434.6	53.2	159.1
Available-for-sale | Non-U.S. governments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	35.2	32.6
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	22.2	15.7	42.8
Transfers out of Level 3 into Level 2	125.3	1.7	87.5
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	26.3	35.4
Transfers out of Level 3 into Level 2	84.5	2.3	7.6
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	183.7
Transfers out of Level 3 into Level 2	101.3
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	0.1
Transfers out of Level 3 into Level 2	88.3	16.6	64.0
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	1.9
Trading | Equity portfolios
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	0.7
Other investments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 1			141.4
Transfers out of Level 3 into Level 2			35.0
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	12.5	45.4	26.9
Transfers out of Level 2 into Level 1	5.9	4.9	8.1
Transfers out of Level 2 into Level 3	3.1	5.3	8.5
Transfers out of Level 3 into Level 2	$ 38.7	$ 0.8	$ 0.3