Investments (Details 5) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Other-Than-Temporary Impairment Credit Losses Recognized in Net Income - Rollforward
Beginning balance	$ (434.9)	$ (325.7)
Credit losses for which an other-than-temporary impairment was not previously recognized	(7.4)	(2.2)
Credit losses for which an other-than-temporary impairment was previously recognized	(20.8)	(34.5)
Reduction for credit losses previously recognized on fixed maturities now sold or intended to be sold	57.4	51.2
Net reduction (increase) for positive changes in cash flows expected to be collected and amortization	1.0	(0.9)
Ending balance	$ (404.7)	$ (312.1)