Investments - Schedule of Amortized Costs and Fair Values of Securities Available for Sale and Gross Unrealized Losses and Fair Values of Available-for-Sale Investments, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt securities:
Total before allowance for credit losses	$ 9,874,543	$ 3,206,120
Gross Unrealized Gains	901,223	153,424
Gross Unrealized Losses	(4,843)	(2,599)
Fair Value	10,770,923	3,356,945
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	174,252	40,550
12 Months or Greater	38,414	29,292
Total	212,666	69,842
Unrealized Losses
Less than 12 Months	(3,836)	(108)
12 Months or Greater	(1,007)	(2,491)
Total	(4,843)	(2,599)
U.S. agencies
Debt securities:
Total before allowance for credit losses	72,945
Gross Unrealized Gains	2,496
Gross Unrealized Losses	0
Fair Value	75,441
Allowance for Credit Losses	0
U.S. Treasury
Debt securities:
Total before allowance for credit losses	3,152
Gross Unrealized Gains	126
Gross Unrealized Losses	0
Fair Value	3,278
Allowance for Credit Losses	0
States and political subdivisions
Debt securities:
Total before allowance for credit losses	528,266	98,037
Gross Unrealized Gains	37,909	4,495
Gross Unrealized Losses	(86)	(3)
Fair Value	566,089	102,529
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	470
12 Months or Greater	1,762	0
Total	1,762	470
Unrealized Losses
Less than 12 Months	0	(3)
12 Months or Greater	(86)	0
Total	(86)	(3)
Foreign governments
Debt securities:
Total before allowance for credit losses	11,115	9,983
Gross Unrealized Gains	334	203
Gross Unrealized Losses	0	0
Fair Value	11,449	10,186
Allowance for Credit Losses	0
Public utilities
Debt securities:
Total before allowance for credit losses	831,990	67,895
Gross Unrealized Gains	77,920	3,476
Gross Unrealized Losses	0	0
Fair Value	909,910	71,371
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	0
12 Months or Greater	0	0
Total	0	0
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	0	0
Total	0	0
Corporate
Debt securities:
Total before allowance for credit losses	7,376,104	2,921,431
Gross Unrealized Gains	727,470	141,705
Gross Unrealized Losses	(4,601)	(2,479)
Fair Value	8,098,973	3,060,657
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	174,252	40,080
12 Months or Greater	36,152	28,582
Total	210,404	68,662
Unrealized Losses
Less than 12 Months	(3,836)	(105)
12 Months or Greater	(765)	(2,374)
Total	(4,601)	(2,479)
Commercial mortgage-backed
Debt securities:
Total before allowance for credit losses	30,108	28,871
Gross Unrealized Gains	1,363	1,071
Gross Unrealized Losses	0	0
Fair Value	31,471	29,942
Allowance for Credit Losses	0
Residential mortgage-backed
Debt securities:
Total before allowance for credit losses	902,974	12,815
Gross Unrealized Gains	50,970	1,077
Gross Unrealized Losses	(156)	(117)
Fair Value	953,788	13,775
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	0
12 Months or Greater	500	710
Total	500	710
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	(156)	(117)
Total	(156)	(117)
Asset-backed
Debt securities:
Total before allowance for credit losses	117,889	67,088
Gross Unrealized Gains	2,635	1,397
Gross Unrealized Losses	0	0
Fair Value	120,524	$ 68,485
Allowance for Credit Losses	$ 0