Investments - Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt securities:
Totals before allowance for credit losses	$ 8,809,594	$ 8,604,250
Gross Unrealized Gains	110,737	482,109
Gross Unrealized Losses	(204,457)	(17,413)
Fair Value	8,715,874	9,068,946
Allowance for Credit Losses	0	0	$ 0	$ 0
Fair Value
Less than 12 Months	2,852,505	816,935
12 Months or Greater	277,504	27,109
Total	3,130,009	844,044
Unrealized Losses
Less than 12 Months	(163,983)	(15,844)
12 Months or Greater	(40,474)	(1,569)
Total	(204,457)	(17,413)
U.S. agencies
Debt securities:
Totals before allowance for credit losses	31,436	43,472
Gross Unrealized Gains	253	1,071
Gross Unrealized Losses	0	0
Fair Value	31,689	44,543
Allowance for Credit Losses	0	0
U.S. Treasury
Debt securities:
Totals before allowance for credit losses	2,473	2,469
Gross Unrealized Gains	6	21
Gross Unrealized Losses	0	0
Fair Value	2,479	2,490
Allowance for Credit Losses	0	0
States and political subdivisions
Debt securities:
Totals before allowance for credit losses	491,323	479,148
Gross Unrealized Gains	9,872	27,733
Gross Unrealized Losses	(17,613)	(921)
Fair Value	483,582	505,960
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	158,698	38,853
12 Months or Greater	14,058	1,790
Total	172,756	40,643
Unrealized Losses
Less than 12 Months	(15,417)	(779)
12 Months or Greater	(2,196)	(142)
Total	(17,613)	(921)
Foreign governments
Debt securities:
Totals before allowance for credit losses	62,977	62,979
Gross Unrealized Gains	12	293
Gross Unrealized Losses	(6,972)	(881)
Fair Value	56,017	62,391
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	48,387	31,862
12 Months or Greater	6,524	0
Total	54,911	31,862
Unrealized Losses
Less than 12 Months	(5,648)	(881)
12 Months or Greater	(1,324)	0
Total	(6,972)	(881)
Public utilities
Debt securities:
Totals before allowance for credit losses	776,673	745,359
Gross Unrealized Gains	6,574	39,919
Gross Unrealized Losses	(13,543)	(309)
Fair Value	769,704	784,969
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	252,793	15,286
12 Months or Greater	538	0
Total	253,331	15,286
Unrealized Losses
Less than 12 Months	(13,439)	(309)
12 Months or Greater	(104)	0
Total	(13,543)	(309)
Corporate
Debt securities:
Totals before allowance for credit losses	6,407,197	6,322,471
Gross Unrealized Gains	89,809	391,287
Gross Unrealized Losses	(136,925)	(12,805)
Fair Value	6,360,081	6,700,953
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	1,689,816	541,974
12 Months or Greater	245,820	25,319
Total	1,935,636	567,293
Unrealized Losses
Less than 12 Months	(101,061)	(11,378)
12 Months or Greater	(35,864)	(1,427)
Total	(136,925)	(12,805)
Commercial mortgage-backed
Debt securities:
Totals before allowance for credit losses	27,001	27,016
Gross Unrealized Gains	13	741
Gross Unrealized Losses	(379)	0
Fair Value	26,635	27,757
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	21,616
12 Months or Greater	0
Total	21,616
Unrealized Losses
Less than 12 Months	(379)
12 Months or Greater	0
Total	(379)
Residential mortgage-backed
Debt securities:
Totals before allowance for credit losses	454,206	530,702
Gross Unrealized Gains	4,047	18,921
Gross Unrealized Losses	(2,448)	0
Fair Value	455,805	549,623
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	197,247
12 Months or Greater	0
Total	197,247
Unrealized Losses
Less than 12 Months	(2,448)
12 Months or Greater	0
Total	(2,448)
Asset-backed
Debt securities:
Totals before allowance for credit losses	556,308	390,634
Gross Unrealized Gains	151	2,123
Gross Unrealized Losses	(26,577)	(2,497)
Fair Value	529,882	390,260
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	483,948	188,960
12 Months or Greater	10,564	0
Total	494,512	188,960
Unrealized Losses
Less than 12 Months	(25,591)	(2,497)
12 Months or Greater	(986)	0
Total	$ (26,577)	$ (2,497)