Separate Accounts, Death Benefits and Other Insurance Benefit Features and Embedded Product Derivatives - GMDB and GMIB Benefits by Type (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Net Amount at Risk by Product and Guarantee [Line Items]
Subtotal separate account liabilities with GMDB	$ 699.2	$ 784.0
Separate account liabilities	1,625.1	1,757.5
Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
General and separate account value	1,741.2	1,879.1
NAR before Reinsurance	77.0	71.9
NAR after Reinsurance	24.3	16.8
General account value	261.7	270.7
Subtotal separate account liabilities with GMDB	1,479.5	1,608.4
Without Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account liabilities	$ 145.6	$ 149.1
Guaranteed Minimum Income Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
Average Attained Age of Annuitant	66 years	65 years
General account value	$ 282.0	$ 308.4
Return On Premium | Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
General and separate account value	575.1	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
Step Up | Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
General and separate account value	$ 1,117.0	$ 1,200.7
NAR before Reinsurance	67.5	65.7
NAR after Reinsurance	$ 14.8	$ 10.6
Average Attained Age of Annuitant	64 years	64 years
Extend Earnings Enhancement Benefit | Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
General and separate account value	$ 27.2	$ 29.1
NAR before Reinsurance	3.2	0.0
NAR after Reinsurance	$ 3.2	$ 0.0
Average Attained Age of Annuitant	65 years	65 years
Extend Annual Step Up or Roll Up | Guaranteed Minimum Death Benefit
Net Amount at Risk by Product and Guarantee [Line Items]
General and separate account value	$ 21.9	$ 22.7
NAR before Reinsurance	5.1	4.8
NAR after Reinsurance	$ 5.1	$ 4.8
Average Attained Age of Annuitant	69 years	69 years