Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Total	$ 836,390	$ 762,855
Fixed Maturities
Cost or Amortized Cost
Due in one year or less	70,131
Due after one year through five years	337,014
Due after five years through ten years	164,523
Due after ten years	12,539
Asset and mortgage backed securities	270,433
Cost or adjusted/amortized cost	$ 854,640	763,434
Percent of Total
Due in one year or less	8.20%
Due after one year through five years	39.40%
Due after five years through ten years	19.30%
Due after ten years	1.50%
Asset and mortgage backed securities	31.60%
Total	100.00%
Fair Value
Due in one year or less	$ 69,785
Due after one year through five years	331,204
Due after five years through ten years	159,481
Due after ten years	12,032
Asset and mortgage backed securities	263,888
Total	$ 836,390	$ 762,855
Percent of Total
Due in one year or less	8.30%
Due after one year through five years	39.60%
Due after five years through ten years	19.10%
Due after ten years	1.40%
Asset and mortgage backed securities	31.60%
Total	100.00%