Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 49,501.3	$ 49,006.7
Fixed maturities, trading	868.7	971.7
Equity securities, trading	536.4	404.8
Other investments	2,952.5	2,985.8
Cash and cash equivalents	1,651.9	2,833.9	1,984.2	1,877.4
Accrued investment income	622.5	615.2
Premiums due and other receivables	1,118.8	1,196.5
Total assets	153,738.2	147,361.7
Deferred income taxes	492.8	208.7
Other liabilities	6,086.5	6,286.2
Total liabilities	143,986.3	137,967.8
Noncontrolling interest	387.0	376.0
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	194.9	214.2
Fixed maturities, trading	132.4	132.4
Equity securities, trading	321.4	207.6
Other investments	90.5	97.8
Cash and cash equivalents	209.1	317.7
Accrued investment income	1.2	1.9
Premiums due and other receivables	56.1	39.1
Total assets	1,005.6	1,010.7
Deferred income taxes	2.1	2.2
Other liabilities	555.4	565.2
Total liabilities	557.5	567.4
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	187.4	199.2
Accrued investment income	0.6	1.2
Total assets	188.0	200.4
Deferred income taxes	2.1	2.2
Other liabilities	130.4	136.9
Total liabilities	132.5	139.1
Collateralized private investment vehicles
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	7.5	15.0
Fixed maturities, trading	132.4	132.4
Accrued investment income	0.1	0.1
Total assets	140.0	147.5
Other liabilities	134.6	143.8
Total liabilities	134.6	143.8
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	89.9	97.5
Accrued investment income	0.5	0.6
Total assets	90.4	98.1
Other liabilities	57.5	64.5
Total liabilities	57.5	64.5
Hedge funds
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, trading	321.4	207.6
Other investments	0.6	0.3
Cash and cash equivalents	209.1	317.7
Premiums due and other receivables	56.1	39.1
Total assets	587.2	564.7
Other liabilities	232.9	220.0
Total liabilities	232.9	220.0
Noncontrolling interest	$ 353.2	$ 343.6