Investments (Available For Sale Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fixed maturities, available for sale
Amortized Cost	$ 2,148,140	$ 2,257,188
Estimated Fair Value	2,125,921	2,280,242
U.S. Treasury obligations
Fixed maturities, available for sale
Amortized Cost	148,588	134,323
Due in one year or less	37,731
Due after one year through five years	84,483
Due after five years through ten years	21,407
Due after ten years	2,836
Estimated Fair Value	146,457	133,627
U.S. Government-sponsored enterprise obligations
Fixed maturities, available for sale
Amortized Cost	32,331	21,089
Due in one year or less	206
Due after one year through five years	11,647
Due after five years through ten years	19,715
Due after ten years	134
Estimated Fair Value	31,702	20,956
State and municipal bonds
Fixed maturities, available for sale
Amortized Cost	326,705	618,414
Due in one year or less	25,562
Due after one year through five years	126,115
Due after five years through ten years	140,281
Due after ten years	39,142
Estimated Fair Value	331,100	632,243
Corporate debt
Fixed maturities, available for sale
Amortized Cost	1,213,362	1,157,660
Due in one year or less	116,559
Due after one year through five years	666,434
Due after five years through ten years	374,029
Due after ten years	38,605
Estimated Fair Value	1,195,627	1,167,158
Residential mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	208,052	196,741
Estimated Fair Value	204,236	197,844
Agency commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	14,080	10,827
Estimated Fair Value	13,812	10,742
Other commercial mortgage-backed securities
Fixed maturities, available for sale
Amortized Cost	33,105	16,004
Estimated Fair Value	32,715	15,961
Other asset-backed securities
Fixed maturities, available for sale
Amortized Cost	171,917	102,130
Estimated Fair Value	$ 170,272	$ 101,711