Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Aggregate consolidated variable interest entities	Dec. 31, 2011 Aggregate consolidated variable interest entities	Dec. 31, 2012 Grantor trusts item	Dec. 31, 2011 Grantor trusts	Dec. 31, 2012 Collateralized private investment vehicles	Dec. 31, 2011 Collateralized private investment vehicles	Dec. 31, 2012 Commercial mortgage-backed securities VIE	Dec. 31, 2011 Commercial mortgage-backed securities VIE	Jun. 30, 2012 Hedge funds	Dec. 31, 2011 Hedge funds
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities							3
Assets of deconsolidated variable interest entities													$ 587.2
Liabilities and noncontrolling interest of deconsolidated variable interest entities													586.1
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	50,939.3	49,006.7			194.6	214.2	194.6	199.2		15.0
Fixed maturities, trading	626.7	971.7			110.4	132.4			110.4	132.4
Equity securities, trading	252.8	404.8			0	207.6								207.6
Other investments	3,291.1	2,985.8			80.3	97.8					80.3	97.5		0.3
Cash and cash equivalents	4,177.2	2,833.9	1,877.4	2,240.4	0	317.7								317.7
Accrued investment income	584.4	615.2			1.1	1.9	0.5	1.2		0.1	0.6	0.6
Premiums due and other receivables	1,084.4	1,196.5				39.1								39.1
Total assets	161,926.5	147,361.7			386.4	1,010.7	195.1	200.4	110.4	147.5	80.9	98.1		564.7
Deferred income taxes	626.5	208.7			1.8	2.2	1.8	2.2
Other liabilities	6,146.1	6,286.2			302.9	565.2	152.4	136.9	104.8	143.8	45.7	64.5		220.0
Total liabilities	152,092.9	137,967.8			304.7	567.4	154.2	139.1	104.8	143.8	45.7	64.5		220.0
Noncontrolling interest	$ 20.0	$ 353.8												$ 343.6