Investments - Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt securities:
Totals before allowance for credit losses	$ 8,723,242	$ 8,604,250
Gross Unrealized Gains	9,152	482,109
Gross Unrealized Losses	(571,442)	(17,413)
Fair Value	8,160,952	9,068,946
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	6,471,084	816,935
12 Months or Greater	264,710	27,109
Total	6,735,794	844,044
Unrealized Losses
Less than 12 Months	(496,919)	(15,844)
12 Months or Greater	(74,523)	(1,569)
Total	(571,442)	(17,413)
U.S. agencies
Debt securities:
Totals before allowance for credit losses	31,289	43,472
Gross Unrealized Gains	23	1,071
Gross Unrealized Losses	(90)	0
Fair Value	31,222	44,543
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	21,185
12 Months or Greater	0
Total	21,185
Unrealized Losses
Less than 12 Months	(90)
12 Months or Greater	0
Total	(90)
U.S. Treasury
Debt securities:
Totals before allowance for credit losses	1,807	2,469
Gross Unrealized Gains	0	21
Gross Unrealized Losses	(4)	0
Fair Value	1,803	2,490
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	1,803
12 Months or Greater	0
Total	1,803
Unrealized Losses
Less than 12 Months	(4)
12 Months or Greater	0
Total	(4)
States and political subdivisions
Debt securities:
Totals before allowance for credit losses	491,283	479,148
Gross Unrealized Gains	1,917	27,733
Gross Unrealized Losses	(43,637)	(921)
Fair Value	449,563	505,960
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	317,551	38,853
12 Months or Greater	12,753	1,790
Total	330,304	40,643
Unrealized Losses
Less than 12 Months	(40,145)	(779)
12 Months or Greater	(3,492)	(142)
Total	(43,637)	(921)
Foreign governments
Debt securities:
Totals before allowance for credit losses	62,973	62,979
Gross Unrealized Gains	1	293
Gross Unrealized Losses	(14,439)	(881)
Fair Value	48,535	62,391
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	41,847	31,862
12 Months or Greater	5,599	0
Total	47,446	31,862
Unrealized Losses
Less than 12 Months	(12,197)	(881)
12 Months or Greater	(2,242)	0
Total	(14,439)	(881)
Public utilities
Debt securities:
Totals before allowance for credit losses	739,559	745,359
Gross Unrealized Gains	576	39,919
Gross Unrealized Losses	(44,322)	(309)
Fair Value	695,813	784,969
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	553,099	15,286
12 Months or Greater	476	0
Total	553,575	15,286
Unrealized Losses
Less than 12 Months	(44,157)	(309)
12 Months or Greater	(165)	0
Total	(44,322)	(309)
Corporate
Debt securities:
Totals before allowance for credit losses	6,372,616	6,322,471
Gross Unrealized Gains	4,734	391,287
Gross Unrealized Losses	(403,546)	(12,805)
Fair Value	5,973,804	6,700,953
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	4,762,723	541,974
12 Months or Greater	232,011	25,319
Total	4,994,734	567,293
Unrealized Losses
Less than 12 Months	(336,373)	(11,378)
12 Months or Greater	(67,173)	(1,427)
Total	(403,546)	(12,805)
Commercial mortgage-backed
Debt securities:
Totals before allowance for credit losses	26,994	27,016
Gross Unrealized Gains	0	741
Gross Unrealized Losses	(932)	0
Fair Value	26,062	27,757
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	26,062
12 Months or Greater	0
Total	26,062
Unrealized Losses
Less than 12 Months	(932)
12 Months or Greater	0
Total	(932)
Residential mortgage-backed
Debt securities:
Totals before allowance for credit losses	382,814	530,702
Gross Unrealized Gains	1,877	18,921
Gross Unrealized Losses	(5,813)	0
Fair Value	378,878	549,623
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	240,379
12 Months or Greater	0
Total	240,379
Unrealized Losses
Less than 12 Months	(5,813)
12 Months or Greater	0
Total	(5,813)
Asset-backed
Debt securities:
Totals before allowance for credit losses	613,907	390,634
Gross Unrealized Gains	24	2,123
Gross Unrealized Losses	(58,659)	(2,497)
Fair Value	555,272	390,260
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	506,435	188,960
12 Months or Greater	13,871	0
Total	520,306	188,960
Unrealized Losses
Less than 12 Months	(57,208)	(2,497)
12 Months or Greater	(1,451)	0
Total	$ (58,659)	$ (2,497)