Fair Value Measurements Derivative Net Realized Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Assets	$ 543.7		$ 543.7		$ 446.6
Derivative Liabilities	380.0		380.0		360.1
Derivative Instruments, Gain (Loss) Recognized in Income, Net	4.5	(166.6)	120.0	(211.2)
Not Designated as Hedging Instrument | Interest rate contracts | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(5.3)	(24.1)	0.7	(53.2)
Not Designated as Hedging Instrument | Foreign exchange contracts | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(11.7)	16.0	1.2	(0.9)
Not Designated as Hedging Instrument | Equity contract | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.7	(1.6)	1.8	(1.3)
Not Designated as Hedging Instrument | Credit contracts
Derivative Instruments, Gain (Loss) [Line Items]
Maximum Potential Future Exposure on Credit Risk Derivatives	339.0		339.0		518.3
Not Designated as Hedging Instrument | Credit contracts | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	3.6	(19.0)	9.8	(13.0)
Not Designated as Hedging Instrument | Managed Custody Guarantees | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0	0.8	1.1	1.3
Investment Contract, with a Fixed Maturity | Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(4.1)	10.7	(4.3)	9.1
Fixed indexed annuities (FIA) | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Assets	0		0		0
Derivative Liabilities	130.5		130.5		236.3
Fixed indexed annuities (FIA) | Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	21.3	(149.6)	109.7	(153.2)
Other liabilities | Not Designated as Hedging Instrument | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	346.4		346.4		306.4
Other liabilities | Not Designated as Hedging Instrument | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	32.9		32.9		32.4
Other liabilities | Not Designated as Hedging Instrument | Equity contract
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	0.1		0.1		0
Other liabilities | Not Designated as Hedging Instrument | Credit contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	$ 0.6		$ 0.6		$ 21.3