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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 30,331		$ 44,437
Held-to-maturity, Allowance for Credit Loss	(11)	$ (14)	(25)	$ (30)	$ (34)	$ (43)
Held to maturity, Gross Unrealized Gains	1,145		1,561
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	31,465		45,973
Carrying Value	30,320		44,412
Total available for sale, Amortized Cost	25,105,981		22,966,462
Available-for-sale, Allowance for Credit Loss	(272)	(318)	(646)	(6,953)	(21,837)	(36,708)
Total available for sale, Gross Unrealized Gains	312,598		96,365
Total available for sale, Gross Unrealized Losses	(500,678)		(708,728)
Total available for sale, Fair Value	24,917,629		22,353,453
Total fixed maturity securities available for sale	24,917,629		22,353,453
Fixed maturity securities, amortized cost	25,136,312		23,010,899
Fixed maturity securities, allowance for credit loss	(283)		(671)
Total investments in fixed maturity securities, Gross Unrealized Gains	313,743		97,926
Total investments in fixed maturity securities, Gross Unrealized Losses	(500,678)		(708,728)
Total investments in fixed maturity securities, Fair Value	24,949,094		22,399,426
Total investments in fixed maturity securities, Carrying Value	24,947,949		22,397,865
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	28,378		42,145
Held-to-maturity, Allowance for Credit Loss	(11)		(25)
Held to maturity, Gross Unrealized Gains	1,068		1,492
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	29,435		43,612
Carrying Value	28,367		42,120
Total available for sale, Amortized Cost	2,004,227		2,376,541
Available-for-sale, Allowance for Credit Loss	0	0	0	0	(339)	(757)
Total available for sale, Gross Unrealized Gains	14,003		7,560
Total available for sale, Gross Unrealized Losses	(44,094)		(89,457)
Total available for sale, Fair Value	1,974,136		2,294,644
Total fixed maturity securities available for sale	1,974,136		2,294,644
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	1,953		2,292
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	77		69
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	2,030		2,361
Carrying Value	1,953		2,292
Total available for sale, Amortized Cost	4,723,950		3,411,796
Available-for-sale, Allowance for Credit Loss	0		(5)
Total available for sale, Gross Unrealized Gains	54,056		11,047
Total available for sale, Gross Unrealized Losses	(140,189)		(189,630)
Total available for sale, Fair Value	4,637,817		3,233,208
Total fixed maturity securities available for sale	4,637,817		3,233,208
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,453,203		2,268,596
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	44,713		9,608
Total available for sale, Gross Unrealized Losses	(19,679)		(42,863)
Total available for sale, Fair Value	3,478,237		2,235,341
Total fixed maturity securities available for sale	3,478,237		2,235,341
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,284,059		1,581,778
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	6,182		3,521
Total available for sale, Gross Unrealized Losses	(31,536)		(67,591)
Total available for sale, Fair Value	1,258,705		1,517,708
Total fixed maturity securities available for sale	1,258,705		1,517,708
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	234,610		272,936
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	3,580		1,439
Total available for sale, Gross Unrealized Losses	(4,760)		(8,981)
Total available for sale, Fair Value	233,430		265,394
Total fixed maturity securities available for sale	233,430		265,394
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	74,625		85,340
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	807		599
Total available for sale, Gross Unrealized Losses	(113)		(347)
Total available for sale, Fair Value	75,319		85,592
Total fixed maturity securities available for sale	75,319		85,592
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	183,872		158,322
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,945		1,079
Total available for sale, Gross Unrealized Losses	(4,325)		(5,827)
Total available for sale, Fair Value	181,492		153,574
Total fixed maturity securities available for sale	181,492		153,574
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	227,061		278,165
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,489		922
Total available for sale, Gross Unrealized Losses	(3,360)		(6,711)
Total available for sale, Fair Value	225,190		272,376
Total fixed maturity securities available for sale	225,190		272,376
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	327,655		534,936
Available-for-sale, Allowance for Credit Loss	0		(425)
Total available for sale, Gross Unrealized Gains	4,123		1,201
Total available for sale, Gross Unrealized Losses	(149)		(3,430)
Total available for sale, Fair Value	331,629		532,282
Total fixed maturity securities available for sale	331,629		532,282
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	5,051,605		3,946,732
Available-for-sale, Allowance for Credit Loss	0	0	(430)	(52)	(1,140)	(158)
Total available for sale, Gross Unrealized Gains	58,179		12,248
Total available for sale, Gross Unrealized Losses	(140,338)		(193,060)
Total available for sale, Fair Value	4,969,446		3,765,490
Total fixed maturity securities available for sale	4,969,446		3,765,490
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,829,765		3,910,363
Available-for-sale, Allowance for Credit Loss	0		0	(546)	(889)	(1,164)
Total available for sale, Gross Unrealized Gains	21,637		16,161
Total available for sale, Gross Unrealized Losses	(20,228)		(41,512)
Total available for sale, Fair Value	3,831,174		3,885,012
Total fixed maturity securities available for sale	3,831,174		3,885,012
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,692,392		3,746,501
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	56,493		14,518
Total available for sale, Gross Unrealized Losses	(42,887)		(93,820)
Total available for sale, Fair Value	3,705,998		3,667,199
Total fixed maturity securities available for sale	3,705,998		3,667,199
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,497,735		3,339,718
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	64,506		18,871
Total available for sale, Gross Unrealized Losses	(17,838)		(38,076)
Total available for sale, Fair Value	3,544,403		3,320,513
Total fixed maturity securities available for sale	3,544,403		3,320,513
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,239,824		795,839
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	21,902		2,970
Total available for sale, Gross Unrealized Losses	(8,995)		(20,115)
Total available for sale, Fair Value	1,252,731		778,694
Total fixed maturity securities available for sale	1,252,731		778,694
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	236,635		653,194
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,562		2,493
Total available for sale, Gross Unrealized Losses	(1,557)		(4,452)
Total available for sale, Fair Value	237,640		651,235
Total fixed maturity securities available for sale	237,640		651,235
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	8,666,586		8,535,252
Available-for-sale, Allowance for Credit Loss	0		0	0		(5,026)
Total available for sale, Gross Unrealized Gains	145,463		38,852
Total available for sale, Gross Unrealized Losses	(71,277)		(156,463)
Total available for sale, Fair Value	8,740,772		8,417,641
Total fixed maturity securities available for sale	8,740,772		8,417,641
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,100,595		1,928,978
Available-for-sale, Allowance for Credit Loss	(272)	$ (318)	(216)	$ (6,355)	$ (19,469)	$ (29,603)
Total available for sale, Gross Unrealized Gains	28,603		11,936
Total available for sale, Gross Unrealized Losses	(205,062)		(185,373)
Total available for sale, Fair Value	1,923,864		1,755,325
Total fixed maturity securities available for sale	$ 1,923,864		$ 1,755,325

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