Investments Classified by Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis, Percent of Total	8.80%
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value, Percent of Total	8.70%
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis, Percent of Total	51.30%
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value, Percent of Total	51.30%
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis, Percent of Total	34.20%
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value, Percent of Total	34.30%
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis, Percent of Total	3.70%
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value, Percent of Total	3.70%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Percent of Total	100.00%
Available-for-sale Securities, Fair Value Disclosure, Percent of Total	100.00%
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 66,969
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	66,739
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	392,256
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	390,907
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	260,758
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	261,919
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	28,549
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	28,385
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	763,434	$ 497,616
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	$ 14,902
Available-for-sale Securities, Debt Maturities, Without Single Maturity Date, Cost, Percent of Total	2.00%
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	$ 14,905
Available-for-sale Securities, Debt Maturities, Without Single Maturity Date, Fair Value, Percent of Total	2.00%
Available-for-sale Securities	$ 762,855	$ 494,516