Investments - Investments Classified by Contractual Maturity Date (Details) - Fixed Maturities - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cost or Amortized Cost
Due in one year or less	$ 50,426
Due after one year through five years	332,620
Due after five years through ten years	192,314
Due after ten years	17,043
Asset and mortgage backed securities	122,971
Cost or adjusted/amortized cost	$ 715,374	$ 497,616
Percent of Total
Due in one year or less	7.00%
Due after one year through five years	46.50%
Due after five years through ten years	26.90%
Due after ten years	2.40%
Asset and mortgage backed securities	17.10%
Total	100.00%
Fair Value
Due in one year or less	$ 50,362
Due after one year through five years	334,135
Due after five years through ten years	194,266
Due after ten years	17,089
Asset and mortgage backed securities	122,933
Total	$ 718,785	$ 494,516
Percent of Total
Due in one year or less	7.00%
Due after one year through five years	46.50%
Due after five years through ten years	27.00%
Due after ten years	2.40%
Asset and mortgage backed securities	17.20%
Total	100.00%