Investments (Available-For-Sale Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Reclassifications from AOCI to net income (loss):
Amortized Cost	$ 2,287,956	$ 2,241,304
Total Fair Value	2,281,664	2,288,785
U.S. Treasury obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	123,840	109,060
Due in one year or less	28,290
Due after one year through five years	79,453
Due after five years through ten years	21,059
Due after ten years	458
Total Fair Value	129,260	110,467
U.S. Government-sponsored enterprise obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	12,205	17,215
Due in one year or less	2,001
Due after one year through five years	5,057
Due after five years through ten years	5,197
Due after ten years	155
Total Fair Value	12,410	17,340
State and municipal bonds
Reclassifications from AOCI to net income (loss):
Amortized Cost	282,451	287,658
Due in one year or less	22,973
Due after one year through five years	113,255
Due after five years through ten years	123,840
Due after ten years	31,599
Total Fair Value	291,667	296,093
Corporate debt
Reclassifications from AOCI to net income (loss):
Amortized Cost	1,302,979	1,308,889
Due in one year or less	130,833
Due after one year through five years	762,741
Due after five years through ten years	371,657
Due after ten years	23,487
Total Fair Value	1,288,718	1,340,364
Residential mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	228,584	205,588
Total Fair Value	230,598	208,408
Agency commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	12,901	8,054
Total Fair Value	13,363	8,221
Other commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	80,470	70,621
Total Fair Value	79,124	71,868
Other asset-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	244,526	234,219
Total Fair Value	$ 236,524	$ 236,024