Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 30,065		$ 44,437
Held-to-maturity, Allowance for Credit Loss	(14)	$ (20)	(25)	$ (34)	$ (38)	$ (43)
Held to maturity, Gross Unrealized Gains	1,114		1,561
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	31,165		45,973
Carrying Value	30,051		44,412
Total available for sale, Amortized Cost	24,339,638		22,966,462
Available-for-sale, Allowance for Credit Loss	(318)	(333)	(646)	(21,837)	(22,831)	(36,708)
Total available for sale, Gross Unrealized Gains	245,445		96,365
Total available for sale, Gross Unrealized Losses	(521,660)		(708,728)
Total available for sale, Fair Value	24,063,105		22,353,453
Total fixed maturity securities available for sale	24,063,105		22,353,453
Fixed maturity securities, amortized cost	24,369,703		23,010,899
Fixed maturity securities, allowance for credit loss	(332)		(671)
Total investments in fixed maturity securities, Gross Unrealized Gains	246,559		97,926
Total investments in fixed maturity securities, Gross Unrealized Losses	(521,660)		(708,728)
Total investments in fixed maturity securities, Fair Value	24,094,270		22,399,426
Total investments in fixed maturity securities, Carrying Value	24,093,156		22,397,865
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	27,983		42,145
Held-to-maturity, Allowance for Credit Loss	(14)		(25)
Held to maturity, Gross Unrealized Gains	1,040		1,492
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	29,009		43,612
Carrying Value	27,969		42,120
Total available for sale, Amortized Cost	2,188,603		2,376,541
Available-for-sale, Allowance for Credit Loss	0	(10)	0	(339)	(693)	(757)
Total available for sale, Gross Unrealized Gains	13,218		7,560
Total available for sale, Gross Unrealized Losses	(55,220)		(89,457)
Total available for sale, Fair Value	2,146,601		2,294,644
Total fixed maturity securities available for sale	2,146,601		2,294,644
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	2,082		2,292
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	74		69
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	2,156		2,361
Carrying Value	2,082		2,292
Total available for sale, Amortized Cost	4,090,672		3,411,796
Available-for-sale, Allowance for Credit Loss	0		(5)
Total available for sale, Gross Unrealized Gains	34,670		11,047
Total available for sale, Gross Unrealized Losses	(157,265)		(189,630)
Total available for sale, Fair Value	3,968,077		3,233,208
Total fixed maturity securities available for sale	3,968,077		3,233,208
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,108,090		2,268,596
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	23,421		9,608
Total available for sale, Gross Unrealized Losses	(30,449)		(42,863)
Total available for sale, Fair Value	3,101,062		2,235,341
Total fixed maturity securities available for sale	3,101,062		2,235,341
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,387,506		1,581,778
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	6,306		3,521
Total available for sale, Gross Unrealized Losses	(40,427)		(67,591)
Total available for sale, Fair Value	1,353,385		1,517,708
Total fixed maturity securities available for sale	1,353,385		1,517,708
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	258,795		272,936
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,817		1,439
Total available for sale, Gross Unrealized Losses	(5,451)		(8,981)
Total available for sale, Fair Value	256,161		265,394
Total fixed maturity securities available for sale	256,161		265,394
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	77,145		85,340
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	555		599
Total available for sale, Gross Unrealized Losses	(223)		(347)
Total available for sale, Fair Value	77,477		85,592
Total fixed maturity securities available for sale	77,477		85,592
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	187,565		158,322
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,688		1,079
Total available for sale, Gross Unrealized Losses	(5,053)		(5,827)
Total available for sale, Fair Value	184,200		153,574
Total fixed maturity securities available for sale	184,200		153,574
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	277,592		278,165
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,852		922
Total available for sale, Gross Unrealized Losses	(4,066)		(6,711)
Total available for sale, Fair Value	275,378		272,376
Total fixed maturity securities available for sale	275,378		272,376
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	355,922		534,936
Available-for-sale, Allowance for Credit Loss	0		(425)
Total available for sale, Gross Unrealized Gains	3,911		1,201
Total available for sale, Gross Unrealized Losses	(333)		(3,430)
Total available for sale, Fair Value	359,500		532,282
Total fixed maturity securities available for sale	359,500		532,282
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,446,594		3,946,732
Available-for-sale, Allowance for Credit Loss	0		(430)	(1,140)	(562)	(158)
Total available for sale, Gross Unrealized Gains	38,581		12,248
Total available for sale, Gross Unrealized Losses	(157,598)		(193,060)
Total available for sale, Fair Value	4,327,577		3,765,490
Total fixed maturity securities available for sale	4,327,577		3,765,490
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,862,993		3,910,363
Available-for-sale, Allowance for Credit Loss	0		0	(889)	(1,097)	(1,164)
Total available for sale, Gross Unrealized Gains	17,247		16,161
Total available for sale, Gross Unrealized Losses	(26,805)		(41,512)
Total available for sale, Fair Value	3,853,435		3,885,012
Total fixed maturity securities available for sale	3,853,435		3,885,012
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,680,399		3,746,501
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	45,729		14,518
Total available for sale, Gross Unrealized Losses	(53,582)		(93,820)
Total available for sale, Fair Value	3,672,546		3,667,199
Total fixed maturity securities available for sale	3,672,546		3,667,199
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,417,541		3,339,718
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	54,356		18,871
Total available for sale, Gross Unrealized Losses	(19,722)		(38,076)
Total available for sale, Fair Value	3,452,175		3,320,513
Total fixed maturity securities available for sale	3,452,175		3,320,513
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,146,714		795,839
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	15,476		2,970
Total available for sale, Gross Unrealized Losses	(10,772)		(20,115)
Total available for sale, Fair Value	1,151,418		778,694
Total fixed maturity securities available for sale	1,151,418		778,694
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	481,868		653,194
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,775		2,493
Total available for sale, Gross Unrealized Losses	(2,006)		(4,452)
Total available for sale, Fair Value	482,637		651,235
Total fixed maturity securities available for sale	482,637		651,235
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	8,726,522		8,535,252
Available-for-sale, Allowance for Credit Loss	0		0	0		(5,026)
Total available for sale, Gross Unrealized Gains	118,336		38,852
Total available for sale, Gross Unrealized Losses	(86,082)		(156,463)
Total available for sale, Fair Value	8,758,776		8,417,641
Total fixed maturity securities available for sale	8,758,776		8,417,641
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,006,836		1,928,978
Available-for-sale, Allowance for Credit Loss	(318)	$ (323)	(216)	$ (19,469)	$ (20,479)	$ (29,603)
Total available for sale, Gross Unrealized Gains	34,642		11,936
Total available for sale, Gross Unrealized Losses	(165,506)		(185,373)
Total available for sale, Fair Value	1,875,654		1,755,325
Total fixed maturity securities available for sale	$ 1,875,654		$ 1,755,325