Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 49,006.7	$ 48,636.3
Fixed maturities, trading	971.7	1,120.3
Equity securities, trading	404.8	316.9
Other investments	2,988.0	2,641.6
Cash and cash equivalents	2,833.9	1,877.4	2,240.4	2,608.0
Accrued investment income	615.2	666.1
Premiums due and other receivables	1,245.2	1,063.0
Total assets	148,298.0	145,631.1
Deferred income taxes	533.4	409.9
Other liabilities	6,286.1	6,143.5
Total liabilities	138,285.0	135,746.1
Noncontrolling interest	376.0	157.2
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	214.2	257.9
Fixed maturities, trading	132.4	131.4
Equity securities, trading	207.6	158.6
Other investments	97.8	128.7
Cash and cash equivalents	317.7	100.0
Accrued investment income	1.9	1.6
Premiums due and other receivables	39.1	15.5
Total assets	1,010.7	793.7
Deferred income taxes	2.2	2.4
Other liabilities	565.2	433.6
Total liabilities	567.4	436.0
Grantor trusts
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities	3
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	199.2	243.1
Accrued investment income	1.2	0.7
Total assets	200.4	243.8
Deferred income taxes	2.2	2.4
Other liabilities	136.9	135.8
Total liabilities	139.1	138.2
Collateralized private investment vehicles
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	15.0	14.8
Fixed maturities, trading	132.4	131.4
Cash and cash equivalents		55.0
Accrued investment income	0.1	0.1
Premiums due and other receivables		1.6
Total assets	147.5	202.9
Other liabilities	143.8	132.6
Total liabilities	143.8	132.6
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	97.5	128.4
Accrued investment income	0.6	0.8
Total assets	98.1	129.2
Other liabilities	64.5	94.1
Total liabilities	64.5	94.1
Hedge funds
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, trading	207.6	158.6
Other investments	0.3	0.3
Cash and cash equivalents	317.7	45.0
Premiums due and other receivables	39.1	13.9
Total assets	564.7	217.8
Other liabilities	220.0	71.1
Total liabilities	220.0	71.1
Noncontrolling interest	$ 343.6	$ 145.9