Guaranteed Benefit Features - Net Amount at Risk of Minimum Guaranteed Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
GMDB
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value	$ 35,117.4	$ 40,629.8
Net amount at risk, net of reinsurance	$ 6,152.3	$ 5,048.3
Weighted average attained age	70 years	70 years
GMAB/GMWB
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value	$ 629.7	$ 775.2
Net amount at risk, net of reinsurance	$ 18.8	$ 16.9
Weighted average attained age	72 years	71 years
GMIB
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value	$ 11,669.3	$ 14,014.9
Net amount at risk, net of reinsurance	$ 3,044.3	$ 2,360.7
Weighted average attained age	63 years	63 years
GMWBL
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value	$ 14,114.2	$ 15,788.3
Net amount at risk, net of reinsurance	$ 2,106.3	$ 1,324.6
Weighted average attained age	67 years	66 years