Investments Maturity Distribution of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Debt Maturities, Due in One Year or Less, Amortized Cost Basis	$ 47,490
Available-for-sale Securities, Debt Maturities, Due After One Year Through Five Years, Amortized Cost Basis	92,764
Available-for-sale Securities, Debt Maturities, Due After Five Years Through Ten Years, Amortized Cost Basis	117,822
Available-for-sale Securities, Debt Maturities, Due After Ten Years, Amortized Cost Basis	662,624
Available-for-sale Debt Securities, Amortized Cost Basis	920,700	$ 860,473
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Debt Maturities, Due in One Year or Less, Fair Value	47,913
Available-for-sale Securities, Debt Maturities, Due After One Year Through Five Years, Fair Value	96,733
Available-for-sale Securities, Debt Maturities, Due After Five Years Through Ten Years, Fair Value	125,611
Available-for-sale Securities, Debt Maturities, Due After Ten Years, Fair Value	684,987
Available-for-sale Debt Securities, Fair Value	955,244
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held-to-maturity Securities, Debt Maturities, Due in One Year or Less, Amortized Cost Basis	20,245
Held-to-maturity Securities, Debt Maturities, Due After One Year Through Five Years, Amortized Cost Basis	44,884
Held-to-maturity Securities, Debt Maturities, Due After Five Years Through Ten Years, Amortized Cost Basis	46,584
Held-to-maturity Securities, Debt Maturities, Due After Ten Years, Amortized Cost Basis	124,640
Held-to-maturity Securities	236,353	247,004
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held-to-maturity Securities, Debt Maturities, Due in One Year or Less, Fair Value	20,446
Held-to-maturity Securities, Debt Maturities, Due After One Year Through Five Years, Fair Value	46,472
Held-to-maturity Securities, Debt Maturities, Due After Five Years Through Ten Years, Fair Value	48,740
Held-to-maturity Securities, Debt Maturities, Due After Ten Years, Fair Value	128,406
Held-to-maturity Securities, Fair Value	244,064	252,545
Total Fixed Maturity Securities, Amortized Cost	1,157,053	1,107,477
Total Fixed Maturity Securities, Fair Value	$ 1,199,308	$ 1,134,213