Fair Value Measurements - Fair Value Measurements - Level 3 unobservable inputs (Details) - Recurring basis - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Significant unobservable inputs used during the valuation of assets categorized within Level 3 of the recurring fair value measurements table
Embedded derivatives - GLWB	$ 10,221	$ 5,712
Level 3
Significant unobservable inputs used during the valuation of assets categorized within Level 3 of the recurring fair value measurements table
Embedded derivatives - GLWB	$ 10,221	$ 5,712
Level 3 | Internal model pricing | Minimum
Unobservable Input
Equity volatility	15.00%	15.00%
Swap curve	1.33%	0.75%
Base Lapse rate	1.00%	1.00%
Level 3 | Internal model pricing | Maximum
Unobservable Input
Equity volatility	28.00%	30.00%
Swap curve	2.54%	3.00%
Base Lapse rate	15.00%	15.00%