Investments Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less, Amortized Cost	$ 44,289
Due after one year through five years, Amortized Cost	429,323
Due after five years though ten years, Amortized Cost	301,211
Due after twenty years	3,025
Due after ten years through twenty years, Amortized Cost	6,677
Asset-backed securities, Amortized Cost	358,138
Totals, Amortized Cost	1,142,663	$ 1,063,971
Fair Value
Due in one year or less, Fair Value	44,539
Due after one year through five years, Fair Value	440,328
Due after five years through ten years, Fair Value	300,043
Due after ten years through twenty years	7,529
Due after twenty years	3,335
Asset-backed securities, Fair Value	358,495
Totals, Fair Value	$ 1,154,269	$ 1,081,637