Investments (Available-For-Sale Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Reclassifications from AOCI to net income (loss):
Amortized Cost	$ 3,666,295	$ 2,361,575
Total Fair Value	3,734,655	2,457,531
U.S. Treasury obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	243,797	104,097
Due in one year or less	13,511
Due after one year through five years	139,736
Due after five years through ten years	90,087
Due after ten years	2,150
Total Fair Value	245,484	107,059
U.S. Government-sponsored enterprise obligations
Reclassifications from AOCI to net income (loss):
Amortized Cost	17,771	12,103
Due in one year or less	3,848
Due after one year through five years	9,137
Due after five years through ten years	4,634
Due after ten years	147
Total Fair Value	17,766	12,261
State and municipal bonds
Reclassifications from AOCI to net income (loss):
Amortized Cost	500,239	316,022
Due in one year or less	14,089
Due after one year through five years	166,059
Due after five years through ten years	175,920
Due after ten years	156,933
Total Fair Value	513,001	332,920
Corporate debt
Reclassifications from AOCI to net income (loss):
Amortized Cost	1,854,526	1,267,992
Due in one year or less	101,331
Due after one year through five years	928,757
Due after five years through ten years	767,911
Due after ten years	99,391
Total Fair Value	1,897,390	1,329,342
Residential mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	479,184	269,752
Total Fair Value	482,255	276,541
Agency commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	15,269	12,623
Total Fair Value	15,759	13,310
Other commercial mortgage-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	170,712	109,244
Total Fair Value	174,241	113,092
Other asset-backed securities
Reclassifications from AOCI to net income (loss):
Amortized Cost	384,797	269,742
Total Fair Value	$ 388,759	$ 273,006