Fair Value Measurements (Details 3) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 19.7	$ 39.9	$ 154.2	$ 86.4
Transfers out of Level 3 into Level 2	32.1	7.0	104.4	129.5
Available-for-sale | Non-U.S. governments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				14.5
Available-for-sale | States and political subdivisions
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		1.8		1.8
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	4.7	28.8	105.3	54.8
Transfers out of Level 3 into Level 2	29.5	2.8	79.6	98.3
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			31.7
Transfers out of Level 3 into Level 2	2.6	4.2	24.8	31.2
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	15.0	9.3	17.2	15.3
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	7.1	7.8	250.9	3,230.6
Transfers out of Level 1 into Level 3			0.1	0.3
Transfers out of Level 2 into Level 1	6.9	0.3	11.7	0.3
Transfers out of Level 2 into Level 3	4.9		12.6	1.3
Transfers out of Level 3 into Level 2	6.3		6.5	1.8
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				$ 9.5