Investments, Contractual Maturities of Available-for-sale Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost [Abstract]
Due in one year or less	$ 50,020
Due after one year through five years	28,979
Due after five years through ten years	28,733
Due after ten years	133,612
Amortized cost	422,255	$ 413,526
Estimated Fair Value [Abstract]
Due in one year or less	49,973
Due after one year through five years	29,299
Due after five years through ten years	29,800
Due after ten years	135,983
Estimated fair value	425,191	411,223
Asset-backed Securities [Member]
Amortized Cost [Abstract]
Without single maturity date	95,223
Amortized cost	95,223	73,317
Estimated Fair Value [Abstract]
Without single maturity date	95,360
Estimated fair value	95,360	73,294
Residential Mortgage-backed Securities [Member]
Amortized Cost [Abstract]
Without single maturity date	85,688
Amortized cost	85,688	84,902
Estimated Fair Value [Abstract]
Without single maturity date	84,776
Estimated fair value	$ 84,776	$ 83,803