Investments - Investments Classified by Contractual Maturity Date (Details) - Fixed Maturities - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017
Cost or Amortized Cost
Due in one year or less	$ 61,999
Due after one year through five years	325,696
Due after five years through ten years	185,127
Due after ten years	15,800
Asset and mortgage backed securities	210,455
Cost or adjusted/amortized cost	$ 799,077	$ 763,434
Percent of Total
Due in one year or less	8.00%
Due after one year through five years	41.00%
Due after five years through ten years	23.00%
Due after ten years	2.00%
Asset and mortgage backed securities	26.00%
Total	100.00%
Fair Value
Due in one year or less				$ 61,715
Due after one year through five years				321,403
Due after five years through ten years				181,628
Due after ten years				15,459
Asset and mortgage backed securities				$ 206,940
Total	$ 787,145	$ 762,855	$ 787,145
Percent of Total
Due in one year or less				8.00%
Due after one year through five years				41.00%
Due after five years through ten years				23.00%
Due after ten years				2.00%
Asset and mortgage backed securities				26.00%
Total				100.00%