Fair Value Measurements (Details 3) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	$ 3,222.8	$ 3,549.7	$ 3,222.8	$ 3,552.0
Transfers out of Level 1 into Level 3			0.3
Transfers out of Level 2 into Level 3	1.3		1.3
Transfers out of Level 3 into Level 2	0.1		1.8
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	22.6		46.5
Transfers out of Level 3 into Level 2	15.0		122.5
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 | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	22.6		26.0
Transfers out of Level 3 into Level 2	15.0		95.5
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2			27.0
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3			6.0
Trading | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 9.5		$ 9.5