Investments - Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
U.S. Treasury obligations
Fixed maturities, available for sale
Amortized Cost	$ 146,186	$ 122,855
Due in one year or less	23,341
Due after one year through five years	104,168
Due after five years through ten years	15,914
Due after ten years	3,116
Total Fair Value	146,539	123,892
U.S. Government-sponsored enterprise obligations
Fixed maturities, available for sale
Amortized Cost	30,038	25,456
Due in one year or less	13,462
Due after one year through five years	4,813
Due after five years through ten years	10,240
Due after ten years	1,720
Total Fair Value	30,235	26,334
State and municipal bonds
Fixed maturities, available for sale
Amortized Cost	790,154	904,719
Due in one year or less	70,859
Due after one year through five years	262,841
Due after five years through ten years	372,660
Due after ten years	94,103
Total Fair Value	800,463	940,635
Corporate debt
Fixed maturities, available for sale
Amortized Cost	1,264,812	1,296,128
Due in one year or less	124,877
Due after one year through five years	743,566
Due after five years through ten years	385,596
Due after ten years	24,952
Total Fair Value	1,278,991	1,291,686
Residential mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	216,285	233,659
Total Fair Value	217,906	238,387
Agency commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	12,837	10,851
Total Fair Value	12,783	10,999
Other commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	19,571	29,983
Total Fair Value	19,611	30,134
Other asset-backed securities
Fixed maturities, available for sale
Amortized Cost	106,938	98,412
Total Fair Value	106,878	98,220
Fixed maturities
Fixed maturities, available for sale
Amortized Cost	2,586,821	2,722,063
Total Fair Value	$ 2,613,406	$ 2,760,287