Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 51,084.1	$ 50,939.3
Fixed maturities, trading	628.9	626.7
Other investments	3,187.2	3,291.1
Accrued investment income	584.5	584.4
Separate account assets	120,906.1	81,653.8
Total assets	200,762.9	161,830.2
Deferred income taxes	791.4	600.0
Separate account liabilities	120,906.1	81,653.8
Other liabilities	6,130.8	6,146.1
Total liabilities	190,877.4	152,066.4
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	220.3	194.6
Fixed maturities, trading	110.4	110.4
Other investments	77.2	80.3
Accrued investment income	1.0	1.1
Separate account assets	33,892.2
Total assets	34,301.1	386.4
Deferred income taxes	1.8	1.8
Separate account liabilities	33,892.2
Other liabilities	318.3	302.9
Total liabilities	34,212.3	304.7
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	220.3	194.6
Accrued investment income	0.5	0.5
Total assets	220.8	195.1
Deferred income taxes	1.8	1.8
Other liabilities	175.3	152.4
Total liabilities	177.1	154.2
Collateralized private investment vehicles
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, trading	110.4	110.4
Total assets	110.4	110.4
Other liabilities	100.4	104.8
Total liabilities	100.4	104.8
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	77.2	80.3
Accrued investment income	0.5	0.6
Total assets	77.7	80.9
Other liabilities	42.6	45.7
Total liabilities	42.6	45.7
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Separate account assets	33,892.2
Total assets	33,892.2
Separate account liabilities	33,892.2
Total liabilities	$ 33,892.2