Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified					3 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2012 Aggregate consolidated variable interest entities	Dec. 31, 2011 Aggregate consolidated variable interest entities	Jun. 30, 2012 Grantor trusts item	Dec. 31, 2011 Grantor trusts	Jun. 30, 2012 Collateralized private investment vehicles	Dec. 31, 2011 Collateralized private investment vehicles	Jun. 30, 2012 Commercial mortgage-backed securities VIE	Dec. 31, 2011 Commercial mortgage-backed securities VIE	Dec. 31, 2011 Hedge funds
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities							3
Decrease in total assets due to deconsolidation					$ 587.2
Decrease in liabilities and noncontrolling interest due to deconsolidation					586.1
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	49,793.6	49,006.7			192.4	214.2	184.8	199.2	7.6	15.0
Fixed maturities, trading	776.5	971.7			132.4	132.4			132.4	132.4
Equity securities, trading	224.3	404.8			0	207.6							207.6
Other investments	3,096.8	2,985.8			83.6	97.8					83.6	97.5	0.3
Cash and cash equivalents	1,646.6	2,833.9	2,169.6	1,877.4	0	317.7							317.7
Accrued investment income	589.5	615.2			1.1	1.9	0.6	1.2		0.1	0.5	0.6
Premiums due and other receivables	1,097.0	1,196.5				39.1							39.1
Total assets	152,050.7	147,361.7			409.5	1,010.7	185.4	200.4	140.0	147.5	84.1	98.1	564.7
Deferred income taxes	568.7	208.7			1.9	2.2	1.9	2.2
Other liabilities	6,011.6	6,286.2			320.7	565.2	133.9	136.9	135.9	143.8	50.9	64.5	220.0
Total liabilities	142,555.3	137,967.8			322.6	567.4	135.8	139.1	135.9	143.8	50.9	64.5	220.0
Noncontrolling interest	$ 12.2	$ 353.8											$ 343.6