Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Total	$ 848,882	$ 762,855
Fixed Maturities
Cost or Amortized Cost
Due in one year or less	60,570
Due after one year through five years	321,349
Due after five years through ten years	179,582
Due after ten years	10,507
Asset and mortgage backed securities	292,313
Cost or adjusted/amortized cost	$ 864,321	763,434
Percent of Total
Due in one year or less	7.00%
Due after one year through five years	37.20%
Due after five years through ten years	20.80%
Due after ten years	1.20%
Asset and mortgage backed securities	33.80%
Total	100.00%
Fair Value
Due in one year or less	$ 60,252
Due after one year through five years	316,312
Due after five years through ten years	174,875
Due after ten years	10,020
Asset and mortgage backed securities	287,423
Total	$ 848,882	$ 762,855
Percent of Total
Due in one year or less	7.10%
Due after one year through five years	37.30%
Due after five years through ten years	20.60%
Due after ten years	1.20%
Asset and mortgage backed securities	33.80%
Total	100.00%