Investments Classified by Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis, Percent of Total	7.40%
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value, Percent of Total	7.50%
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis, Percent of Total	55.00%
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value, Percent of Total	55.10%
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis, Percent of Total	28.60%
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value, Percent of Total	28.40%
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis, Percent of Total	9.00%
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value, Percent of Total	9.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Percent of Total	100.00%
Available-for-sale Securities	$ 528,647	$ 452,714
Available-for-sale Securities, Fair Value Disclosure, Percent of Total	100.00%
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 36,973
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	36,974
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	273,784
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	272,822
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	142,452
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	140,416
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	44,407
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	44,304
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	497,616	396,415
Available-for-sale Securities	$ 494,516	$ 396,698