Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 50,048.1	$ 48,757.1
Fixed maturities, trading	575.3	563.1
Equity securities, trading	724.2	716.9
Other investments	2,910.4	2,944.4
Accrued investment income	540.4	532.1
Separate account assets	132,804.6	130,018.4
Total assets	211,193.9	208,191.4
Deferred income taxes	889.7	824.0
Separate account liabilities	132,804.6	130,018.4
Other liabilities	4,936.2	5,224.2
Total liabilities	200,855.8	198,167.2
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	279.5	272.0
Fixed maturities, trading	110.4	110.4
Equity securities, trading	325.5	327.2
Other investments	57.9	68.1
Accrued investment income	0.8	0.9
Separate account assets	32,652.9	32,824.7
Total assets	33,427.0	33,603.3
Deferred income taxes	1.7	1.5
Separate account liabilities	32,652.9	32,824.7
Other liabilities	360.2	342.4
Total liabilities	33,014.8	33,168.6
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	279.5	272.0
Accrued investment income	0.4	0.3
Total assets	279.9	272.3
Deferred income taxes	1.7	1.5
Other liabilities	228.9	217.2
Total liabilities	230.6	218.7
Collateralized private investment vehicle
Consolidated Variable Interest Entity disclosures
Number of entities for which the reporting entity acts as primary beneficiary	1
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, trading	110.4	110.4
Total assets	110.4	110.4
Other liabilities	109.9	93.8
Total liabilities	109.9	93.8
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	57.9	68.1
Accrued investment income	0.4	0.6
Total assets	58.3	68.7
Other liabilities	21.4	31.4
Total liabilities	21.4	31.4
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, trading	325.5	327.2
Separate account assets	32,652.9	32,824.7
Total assets	32,978.4	33,151.9
Separate account liabilities	32,652.9	32,824.7
Total liabilities	$ 32,652.9	$ 32,824.7