Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 44,939	$ 44,437
Held-to-maturity, Allowance for Credit Loss	(20)	(25)	$ (38)	$ (43)
Held to maturity, Gross Unrealized Gains	1,298	1,561
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	46,217	45,973
Carrying Value	44,919	44,412
Total available for sale, Amortized Cost	24,005,146	22,966,462
Available-for-sale, Allowance for Credit Loss	(333)	(646)	(22,831)	(36,708)
Total available for sale, Gross Unrealized Gains	169,114	96,365
Total available for sale, Gross Unrealized Losses	(598,042)	(708,728)
Total available for sale, Fair Value	23,575,885	22,353,453
Total fixed maturity securities available for sale	23,575,885	22,353,453
Fixed maturity securities, amortized cost	24,050,085	23,010,899
Fixed maturity securities, allowance for credit loss	(353)	(671)
Total investments in fixed maturity securities, Gross Unrealized Gains	170,412	97,926
Total investments in fixed maturity securities, Gross Unrealized Losses	(598,042)	(708,728)
Total investments in fixed maturity securities, Fair Value	23,622,102	22,399,426
Total investments in fixed maturity securities, Carrying Value	23,620,804	22,397,865
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	42,738	42,145
Held-to-maturity, Allowance for Credit Loss	(20)	(25)
Held to maturity, Gross Unrealized Gains	1,219	1,492
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	43,937	43,612
Carrying Value	42,718	42,120
Total available for sale, Amortized Cost	2,249,284	2,376,541
Available-for-sale, Allowance for Credit Loss	(10)	0	(693)	(757)
Total available for sale, Gross Unrealized Gains	11,880	7,560
Total available for sale, Gross Unrealized Losses	(65,121)	(89,457)
Total available for sale, Fair Value	2,196,033	2,294,644
Total fixed maturity securities available for sale	2,196,033	2,294,644
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	2,201	2,292
Held-to-maturity, Allowance for Credit Loss	0	0
Held to maturity, Gross Unrealized Gains	79	69
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	2,280	2,361
Carrying Value	2,201	2,292
Total available for sale, Amortized Cost	3,794,761	3,411,796
Available-for-sale, Allowance for Credit Loss	0	(5)
Total available for sale, Gross Unrealized Gains	29,012	11,047
Total available for sale, Gross Unrealized Losses	(159,398)	(189,630)
Total available for sale, Fair Value	3,664,375	3,233,208
Total fixed maturity securities available for sale	3,664,375	3,233,208
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,933,935	2,268,596
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	21,138	9,608
Total available for sale, Gross Unrealized Losses	(28,095)	(42,863)
Total available for sale, Fair Value	2,926,978	2,235,341
Total fixed maturity securities available for sale	2,926,978	2,235,341
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,479,689	1,581,778
Available-for-sale, Allowance for Credit Loss	(10)	0
Total available for sale, Gross Unrealized Gains	6,122	3,521
Total available for sale, Gross Unrealized Losses	(49,162)	(67,591)
Total available for sale, Fair Value	1,436,639	1,517,708
Total fixed maturity securities available for sale	1,436,639	1,517,708
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	260,503	272,936
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,382	1,439
Total available for sale, Gross Unrealized Losses	(6,997)	(8,981)
Total available for sale, Fair Value	255,888	265,394
Total fixed maturity securities available for sale	255,888	265,394
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	77,493	85,340
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	618	599
Total available for sale, Gross Unrealized Losses	(218)	(347)
Total available for sale, Fair Value	77,893	85,592
Total fixed maturity securities available for sale	77,893	85,592
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	157,705	158,322
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,214	1,079
Total available for sale, Gross Unrealized Losses	(4,207)	(5,827)
Total available for sale, Fair Value	154,712	153,574
Total fixed maturity securities available for sale	154,712	153,574
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	273,894	278,165
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,544	922
Total available for sale, Gross Unrealized Losses	(4,537)	(6,711)
Total available for sale, Fair Value	270,901	272,376
Total fixed maturity securities available for sale	270,901	272,376
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	428,726	534,936
Available-for-sale, Allowance for Credit Loss	0	(425)
Total available for sale, Gross Unrealized Gains	2,973	1,201
Total available for sale, Gross Unrealized Losses	(861)	(3,430)
Total available for sale, Fair Value	430,838	532,282
Total fixed maturity securities available for sale	430,838	532,282
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,223,487	3,946,732
Available-for-sale, Allowance for Credit Loss	0	(430)	(562)	(158)
Total available for sale, Gross Unrealized Gains	31,985	12,248
Total available for sale, Gross Unrealized Losses	(160,259)	(193,060)
Total available for sale, Fair Value	4,095,213	3,765,490
Total fixed maturity securities available for sale	4,095,213	3,765,490
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,991,472	3,910,363
Available-for-sale, Allowance for Credit Loss	0	0	(1,097)	(1,164)
Total available for sale, Gross Unrealized Gains	14,188	16,161
Total available for sale, Gross Unrealized Losses	(33,989)	(41,512)
Total available for sale, Fair Value	3,971,671	3,885,012
Total fixed maturity securities available for sale	3,971,671	3,885,012
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,754,016	3,746,501
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	29,080	14,518
Total available for sale, Gross Unrealized Losses	(71,968)	(93,820)
Total available for sale, Fair Value	3,711,128	3,667,199
Total fixed maturity securities available for sale	3,711,128	3,667,199
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,404,293	3,339,718
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	34,749	18,871
Total available for sale, Gross Unrealized Losses	(26,872)	(38,076)
Total available for sale, Fair Value	3,412,170	3,320,513
Total fixed maturity securities available for sale	3,412,170	3,320,513
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	945,725	795,839
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	8,248	2,970
Total available for sale, Gross Unrealized Losses	(14,619)	(20,115)
Total available for sale, Fair Value	939,354	778,694
Total fixed maturity securities available for sale	939,354	778,694
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	498,350	653,194
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,242	2,493
Total available for sale, Gross Unrealized Losses	(2,886)	(4,452)
Total available for sale, Fair Value	497,706	651,235
Total fixed maturity securities available for sale	497,706	651,235
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	8,602,384	8,535,252
Available-for-sale, Allowance for Credit Loss	0	0	0	(5,026)
Total available for sale, Gross Unrealized Gains	74,319	38,852
Total available for sale, Gross Unrealized Losses	(116,345)	(156,463)
Total available for sale, Fair Value	8,560,358	8,417,641
Total fixed maturity securities available for sale	8,560,358	8,417,641
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,004,584	1,928,978
Available-for-sale, Allowance for Credit Loss	(323)	(216)	$ (20,479)	$ (29,603)
Total available for sale, Gross Unrealized Gains	15,604	11,936
Total available for sale, Gross Unrealized Losses	(194,233)	(185,373)
Total available for sale, Fair Value	1,825,632	1,755,325
Total fixed maturity securities available for sale	$ 1,825,632	$ 1,755,325