Investments - Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt securities:
Totals before allowance for credit losses	$ 8,083,487	$ 8,438,760
Gross Unrealized Gains	4,014	3,082
Gross Unrealized Losses	(763,401)	(830,209)
Fair Value	7,324,100	7,611,633
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	1,004,269	6,673,701
12 Months or Greater	6,067,818	653,807
Total	7,072,087	7,327,508
Unrealized Losses
Less than 12 Months	(19,086)	(635,555)
12 Months or Greater	(744,315)	(194,656)
Total	(763,401)	(830,211)
U.S. agencies
Debt securities:
Totals before allowance for credit losses	25,563	21,003
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(258)	(377)
Fair Value	25,305	20,626
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	4,812	20,626
12 Months or Greater	20,493	0
Total	25,305	20,626
Unrealized Losses
Less than 12 Months	(23)	(377)
12 Months or Greater	(235)	0
Total	(258)	(377)
U.S. Treasury
Debt securities:
Totals before allowance for credit losses	2,817	2,813
Gross Unrealized Gains	0	6
Gross Unrealized Losses	(97)	(90)
Fair Value	2,720	2,729
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	1,005	1,749
12 Months or Greater	1,715	0
Total	2,720	1,749
Unrealized Losses
Less than 12 Months	(6)	(90)
12 Months or Greater	(91)	0
Total	(97)	(90)
States and political subdivisions
Debt securities:
Totals before allowance for credit losses	459,815	476,338
Gross Unrealized Gains	535	668
Gross Unrealized Losses	(54,711)	(65,507)
Fair Value	405,639	411,499
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	73,667	346,009
12 Months or Greater	298,198	28,420
Total	371,865	374,429
Unrealized Losses
Less than 12 Months	(1,552)	(55,014)
12 Months or Greater	(53,159)	(10,493)
Total	(54,711)	(65,507)
Foreign governments
Debt securities:
Totals before allowance for credit losses	62,956	62,964
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15,056)	(17,076)
Fair Value	47,900	45,888
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	1,027	22,591
12 Months or Greater	46,873	23,296
Total	47,900	45,887
Unrealized Losses
Less than 12 Months	(38)	(7,658)
12 Months or Greater	(15,018)	(9,420)
Total	(15,056)	(17,078)
Public utilities
Debt securities:
Totals before allowance for credit losses	618,638	681,785
Gross Unrealized Gains	113	117
Gross Unrealized Losses	(63,324)	(66,765)
Fair Value	555,427	615,137
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	71,438	601,824
12 Months or Greater	481,425	10,747
Total	552,863	612,571
Unrealized Losses
Less than 12 Months	(483)	(61,970)
12 Months or Greater	(62,841)	(4,795)
Total	(63,324)	(66,765)
Corporate
Debt securities:
Totals before allowance for credit losses	5,711,727	6,199,886
Gross Unrealized Gains	1,680	1,940
Gross Unrealized Losses	(523,221)	(570,255)
Fair Value	5,190,186	5,631,571
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	620,229	4,985,075
12 Months or Greater	4,457,994	434,625
Total	5,078,223	5,419,700
Unrealized Losses
Less than 12 Months	(11,013)	(432,492)
12 Months or Greater	(512,208)	(137,763)
Total	(523,221)	(570,255)
Commercial mortgage-backed
Debt securities:
Totals before allowance for credit losses	26,950	21,965
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,081)	(1,680)
Fair Value	24,869	20,285
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	4,964	20,285
12 Months or Greater	19,905	0
Total	24,869	20,285
Unrealized Losses
Less than 12 Months	(37)	(1,680)
12 Months or Greater	(2,044)	0
Total	(2,081)	(1,680)
Residential mortgage-backed
Debt securities:
Totals before allowance for credit losses	284,705	337,186
Gross Unrealized Gains	179	183
Gross Unrealized Losses	(15,441)	(16,338)
Fair Value	269,443	321,031
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	77,817	307,410
12 Months or Greater	186,047	0
Total	263,864	307,410
Unrealized Losses
Less than 12 Months	(2,094)	(16,338)
12 Months or Greater	(13,347)	0
Total	(15,441)	(16,338)
Asset-backed
Debt securities:
Totals before allowance for credit losses	890,316	634,820
Gross Unrealized Gains	1,507	168
Gross Unrealized Losses	(89,212)	(92,121)
Fair Value	802,611	542,867
Allowance for Credit Losses	0	0
Fair Value
Less than 12 Months	149,310	368,132
12 Months or Greater	555,168	156,719
Total	704,478	524,851
Unrealized Losses
Less than 12 Months	(3,840)	(59,936)
12 Months or Greater	(85,372)	(32,185)
Total	$ (89,212)	$ (92,121)