Investments - Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fixed maturities, available for sale
Amortized Cost	$ 2,361,575	$ 2,241,304
Total Fair Value	2,457,531	2,288,785
U.S. Treasury obligations
Fixed maturities, available for sale
Amortized Cost	104,097	109,060
Due in one year or less	23,049
Due after one year through five years	73,580
Due after five years through ten years	10,430
Due after ten years	0
Total Fair Value	107,059	110,467
U.S. Government-sponsored enterprise obligations
Fixed maturities, available for sale
Amortized Cost	12,103	17,215
Due in one year or less	0
Due after one year through five years	9,093
Due after five years through ten years	3,014
Due after ten years	154
Total Fair Value	12,261	17,340
State and municipal bonds
Fixed maturities, available for sale
Amortized Cost	316,022	287,658
Due in one year or less	14,466
Due after one year through five years	141,826
Due after five years through ten years	158,927
Due after ten years	17,701
Total Fair Value	332,920	296,093
Corporate debt
Fixed maturities, available for sale
Amortized Cost	1,267,992	1,308,889
Due in one year or less	143,719
Due after one year through five years	700,708
Due after five years through ten years	425,711
Due after ten years	59,204
Total Fair Value	1,329,342	1,340,364
Residential mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	269,752	205,588
Total Fair Value	276,541	208,408
Agency commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	12,623	8,054
Total Fair Value	13,310	8,221
Other commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	109,244	70,621
Total Fair Value	113,092	71,868
Other asset-backed securities
Fixed maturities, available for sale
Amortized Cost	269,742	234,219
Total Fair Value	$ 273,006	$ 236,024