Investments (Available-For-Sale Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Reclassifications from AOCI to net income (loss):
Amortized Cost	$ 2,182,473	$ 2,241,304
Total Fair Value	2,259,948	2,288,785
U.S. Treasury obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	110,405	109,060
Due in one year or less	32,905
Due after one year through five years	63,387
Due after five years through ten years	18,949
Due after ten years	0
Total Fair Value	115,241	110,467
U.S. Government-sponsored enterprise obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	8,506	17,215
Due in one year or less	0
Due after one year through five years	4,929
Due after five years through ten years	3,606
Due after ten years	158
Total Fair Value	8,693	17,340
State and municipal bonds
Reclassifications from AOCI to net income (loss):
Amortized Cost	277,091	287,658
Due in one year or less	18,435
Due after one year through five years	117,564
Due after five years through ten years	128,996
Due after ten years	28,723
Total Fair Value	293,718	296,093
Corporate debt
Reclassifications from AOCI to net income (loss):
Amortized Cost	1,229,377	1,308,889
Due in one year or less	132,517
Due after one year through five years	726,096
Due after five years through ten years	385,178
Due after ten years	30,652
Total Fair Value	1,274,443	1,340,364
Residential mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	227,593	205,588
Total Fair Value	234,693	208,408
Agency commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	12,450	8,054
Total Fair Value	13,192	8,221
Other commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	88,852	70,621
Total Fair Value	90,757	71,868
Other asset-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	228,199	234,219
Total Fair Value	$ 229,211	$ 236,024