Variable Interest Entities - Consolidated VIEs (Details) $ in Millions	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 49,966.5	$ 49,670.8
Fixed maturities, trading	686.8	604.6
Equity securities, available-for-sale	104.5	123.0
Equity securities, trading	1,202.7	840.2
Real estate	1,451.8	1,344.6
Other investments	3,251.7	3,209.8
Accrued investment income	545.6	505.9
Premiums due and other receivables	1,429.3	1,213.0
Separate account assets	136,978.9	140,072.8
Other assets	1,068.7	1,083.5
Total assets	218,685.9	219,087.0
Contractholder funds	35,716.1	34,726.7
Long-term debt	3,290.8	2,531.2
Income taxes currently payable	18.4	11.5
Deferred income taxes	697.2	1,035.3
Separate account liabilities	136,978.9	140,072.8
Other liabilities	5,678.4	5,542.2
Total liabilities	209,222.8	208,797.0
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	257.4	278.2
Fixed maturities, trading	100.4	100.4
Equity securities, trading	640.9	345.3
Real estate	354.5	284.9
Other investments	29.5	40.6
Cash	15.3	4.7
Accrued investment income	3.1	2.0
Premiums due and other receivables	1.6
Separate account assets	33,300.4	34,655.4
Other assets	(0.9)	0.3
Total assets	34,702.2	35,711.8
Contractholder funds	338.9
Long-term debt	42.8	82.3
Income taxes currently payable	0.5	10.6
Deferred income taxes	(0.2)	1.1
Separate account liabilities	33,300.4	34,655.4
Other liabilities	345.9	344.0
Total liabilities	$ 34,028.3	35,093.4
Grantor trusts
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities | item	3
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 257.4	278.2
Accrued investment income	0.5	0.4
Total assets	257.9	278.6
Deferred income taxes	1.5	1.5
Other liabilities	230.3	239.1
Total liabilities	$ 231.8	240.6
Collateralized private investment vehicle
Consolidated Variable Interest Entity disclosures
Number of entities for which the reporting entity acts as primary beneficiary | item	1
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, trading	$ 100.4	100.4
Total assets	100.4	100.4
Other liabilities	85.9	85.6
Total liabilities	85.9	85.6
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	18.3	35.0
Accrued investment income	0.1	0.2
Total assets	18.4	35.2
Other liabilities		4.8
Total liabilities		4.8
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, trading	640.9	345.3
Separate account assets	33,300.4	34,655.4
Total assets	33,941.3	35,000.7
Contractholder funds	338.9
Separate account liabilities	33,300.4	34,655.4
Total liabilities	33,639.3	34,655.4
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Real estate	354.5	284.9
Other investments	11.2	5.6
Cash	15.3	4.7
Accrued investment income	2.5	1.4
Premiums due and other receivables	1.6
Other assets	(0.9)	0.3
Total assets	384.2	296.9
Long-term debt	42.8	82.3
Income taxes currently payable	0.5	10.6
Deferred income taxes	(1.7)	(0.4)
Other liabilities	29.7	14.5
Total liabilities	$ 71.3	$ 107.0