Separate Accounts, Death Benefits and Other Insurance Benefit Features and Embedded Product Derivatives - GMDB and GMIB Benefits by Type (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	$ 1,826.1	$ 1,879.1
NAR before Reinsurance	66.8	71.9
NAR after Reinsurance	13.9	16.8
Average Attained Age of Annuitant	65 years	65 years
Less: General account value with GMDB	267.1	270.7
Subtotal separate account liabilities with GMDB	1,559.0	1,608.4
Separate account liabilities without GMDB	148.9	149.1
Total separate account liabilities	1,707.9	1,757.5
GMIB	297.5	308.4
GMDB return of premium
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	604.6	626.6
NAR before Reinsurance	1.2	1.4
NAR after Reinsurance	1.2	1.4
Average Attained Age of Annuitant	64 years	63 years
GMDB step up
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	1,170.1	1,200.7
NAR before Reinsurance	60.8	65.7
NAR after Reinsurance	7.9	10.6
Average Attained Age of Annuitant	65 years	64 years
GMDB earnings enhancement benefit (“EEB”)
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	29.0	29.1
NAR before Reinsurance	0	0
NAR after Reinsurance	0	0
Average Attained Age of Annuitant	65 years	65 years
GMDB greater of annual step up and roll up
Net Amount at Risk by Product and Guarantee [Line Items]
Account Value	22.4	22.7
NAR before Reinsurance	4.8	4.8
NAR after Reinsurance	$ 4.8	$ 4.8
Average Attained Age of Annuitant	69 years	69 years