Summary of Significant Accounting Policies - Guaranteed Minimum Death Benefit (Details) - Guarantee Minimum Death Benefit - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net Amount at Risk by Product and Guarantee [Line Items]
GMDB reserve	$ 152	$ 225
Aggregate in-the-money death benefits under the GMDB provision	$ 28,345	$ 32,106
Variable annuity contract value distribution based on GMDB feature:
No guarantee	29.00%	32.00%
Return of premium guarantee	65.00%	62.00%
Guarantee of premium roll-up at an annual rate of 3% or 5%	6.00%	6.00%
Total	100.00%	100.00%