Fair Value Measurements - Level 3 unobservable inputs (Details) - Recurring basis - Level 3 - Internal model pricing	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2016
Minimum
Unobservable Input
Equity volatility	15.00%	15.00%
Swap curve	2.31%	1.69%
Base Lapse rate	1.00%	1.00%
Maximum
Unobservable Input
Equity volatility	30.00%	30.00%
Swap curve	2.85%	2.54%
Base Lapse rate	15.00%	15.00%