Minimum Guarantees - Separate Account Liabilities for GMDB (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account liability	$ 685,651	$ 469,176
Net amount at risk, net of reinsurance	19,185	28,832
GMDB
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account liability	64,953	55,607
Net amount at risk, net of reinsurance	$ 17,705	$ 25,891
Weighted average attained age	70 years	71 years
GLWB
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account liability	$ 620,698	$ 413,569
Net amount at risk, net of reinsurance	$ 1,480	$ 2,941
Weighted average attained age	69 years	58 years