Variable Interest Entities (Details) $ in Millions	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 49,780.6	$ 49,670.8
Fixed maturities, trading	710.2	604.6
Equity securities, available-for-sale	465.3	123.0
Equity securities, trading	869.1	840.2
Real estate	1,415.0	1,344.6
Other investments	3,270.4	3,209.8
Accrued investment income	566.8	505.9
Premiums due and other receivables	1,401.6	1,213.0
Separate account assets	134,045.8	140,072.8
Other assets	1,110.8	1,083.5
Total assets	215,365.4	219,087.0
Contractholder funds	35,115.2	34,726.7
Long-term debt	3,286.1	2,531.2
Income taxes currently payable	16.7	11.5
Deferred income taxes	878.8	1,035.3
Separate account liabilities	134,045.8	140,072.8
Other liabilities	5,696.2	5,542.2
Total liabilities	205,601.6	208,797.0
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	261.1	278.2
Fixed maturities, trading	100.4	100.4
Equity securities, available-for-sale	315.8
Equity securities, trading	321.2	345.3
Real estate	298.9	284.9
Other investments	36.8	40.6
Cash	7.8	4.7
Accrued investment income	2.5	2.0
Premiums due and other receivables	2.0
Separate account assets	32,827.6	34,655.4
Other assets	(0.9)	0.3
Total assets	34,173.2	35,711.8
Contractholder funds	329.2
Long-term debt	38.2	82.3
Income taxes currently payable	(0.1)	10.6
Deferred income taxes	0.6	1.1
Separate account liabilities	32,827.6	34,655.4
Other liabilities	340.7	344.0
Total liabilities	$ 33,536.2	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	$ 261.1	278.2
Accrued investment income	0.4	0.4
Total assets	261.5	278.6
Deferred income taxes	1.6	1.5
Other liabilities	236.7	239.1
Total liabilities	$ 238.3	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	87.2	85.6
Total liabilities	87.2	85.6
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	25.1	35.0
Accrued investment income	0.2	0.2
Total assets	25.3	35.2
Other liabilities		4.8
Total liabilities		4.8
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, available-for-sale	315.8
Equity securities, trading	321.2	345.3
Separate account assets	32,827.6	34,655.4
Total assets	33,464.6	35,000.7
Contractholder funds	329.2
Separate account liabilities	32,827.6	34,655.4
Total liabilities	33,156.8	34,655.4
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Real estate	298.9	284.9
Other investments	11.7	5.6
Cash	7.8	4.7
Accrued investment income	1.9	1.4
Premiums due and other receivables	2.0
Other assets	(0.9)	0.3
Total assets	321.4	296.9
Long-term debt	38.2	82.3
Income taxes currently payable	(0.1)	10.6
Deferred income taxes	(1.0)	(0.4)
Other liabilities	16.8	14.5
Total liabilities	$ 53.9	$ 107.0