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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 54,020	$ 53,415
Held-to-maturity, Allowance for Credit Loss	(38)	(43)	$ (107)	$ (114)
Held to maturity, Gross Unrealized Gains	2,358	3,239
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	56,340	56,611
Total held to maturity, Carrying Value	53,982	53,372
Total available for sale, Amortized Cost	21,721,496	20,861,830
Available-for-sale, Allowance for Credit Loss	(22,831)	(36,708)	(37,142)	(37,352)
Total available for sale, Gross Unrealized Gains	73,248	110,989
Total available for sale, Gross Unrealized Losses	(861,041)	(811,175)
Total available for sale, Fair Value	20,910,872	20,124,936
Total fixed maturity securities available for sale	20,910,872	20,124,936
Total investments in fixed maturity securities, Amortized Value	21,775,516	20,915,245
Fixed maturity securities, allowance for credit loss	(22,869)	(36,751)
Total investments in fixed maturity securities, Gross Unrealized Gains	75,606	114,228
Total investments in fixed maturity securities, Gross Unrealized Losses	(861,041)	(811,175)
Total investments in fixed maturity securities, Fair Value	20,967,212	20,181,547
Total investments in fixed maturity securities, Carrying Value	20,964,854	20,178,308
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	51,258	50,547
Held-to-maturity, Allowance for Credit Loss	(38)	(43)
Held to maturity, Gross Unrealized Gains	2,276	3,132
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	53,496	53,636
Total held to maturity, Carrying Value	51,220	50,504
Total available for sale, Amortized Cost	2,668,420	2,742,922
Available-for-sale, Allowance for Credit Loss	(693)	(757)
Total available for sale, Gross Unrealized Gains	8,948	14,719
Total available for sale, Gross Unrealized Losses	(123,889)	(122,462)
Total available for sale, Fair Value	2,552,786	2,634,422
Total fixed maturity securities available for sale	2,552,786	2,634,422
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	2,762	2,868
Held-to-maturity, Allowance for Credit Loss	0	0
Held to maturity, Gross Unrealized Gains	82	107
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	2,844	2,975
Total held to maturity, Carrying Value	2,762	2,868
Total available for sale, Amortized Cost	1,963,060	1,773,206
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	9,805	12,780
Total available for sale, Gross Unrealized Losses	(174,727)	(163,844)
Total available for sale, Fair Value	1,798,138	1,622,142
Total fixed maturity securities available for sale	1,798,138	1,622,142
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,875,920	1,762,997
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	8,888	11,403
Total available for sale, Gross Unrealized Losses	(55,256)	(57,669)
Total available for sale, Fair Value	1,829,552	1,716,731
Total fixed maturity securities available for sale	1,829,552	1,716,731
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,626,511	1,682,550
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	3,370	5,651
Total available for sale, Gross Unrealized Losses	(83,769)	(82,006)
Total available for sale, Fair Value	1,546,112	1,606,195
Total fixed maturity securities available for sale	1,546,112	1,606,195
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	393,807	394,429
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,833	3,550
Total available for sale, Gross Unrealized Losses	(16,316)	(16,405)
Total available for sale, Fair Value	379,324	381,574
Total fixed maturity securities available for sale	379,324	381,574
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	88,359	103,029
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	908	1,634
Total available for sale, Gross Unrealized Losses	(214)	(185)
Total available for sale, Fair Value	89,053	104,478
Total fixed maturity securities available for sale	89,053	104,478
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	166,261	166,873
Available-for-sale, Allowance for Credit Loss	(693)	(757)
Total available for sale, Gross Unrealized Gains	539	696
Total available for sale, Gross Unrealized Losses	(10,588)	(11,973)
Total available for sale, Fair Value	155,519	154,839
Total fixed maturity securities available for sale	155,519	154,839
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	393,482	396,041
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,298	3,188
Total available for sale, Gross Unrealized Losses	(13,002)	(11,893)
Total available for sale, Fair Value	382,778	387,336
Total fixed maturity securities available for sale	382,778	387,336
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	630,348	657,157
Available-for-sale, Allowance for Credit Loss	(562)	(158)
Total available for sale, Gross Unrealized Gains	326	626
Total available for sale, Gross Unrealized Losses	(20,232)	(13,312)
Total available for sale, Fair Value	609,880	644,313
Total fixed maturity securities available for sale	609,880	644,313
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,593,408	2,430,363
Available-for-sale, Allowance for Credit Loss	(562)	(158)	(23)	(18)
Total available for sale, Gross Unrealized Gains	10,131	13,406
Total available for sale, Gross Unrealized Losses	(194,959)	(177,156)
Total available for sale, Fair Value	2,408,018	2,266,455
Total fixed maturity securities available for sale	2,408,018	2,266,455
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,298,768	4,252,883
Available-for-sale, Allowance for Credit Loss	(1,097)	(1,164)
Total available for sale, Gross Unrealized Gains	6,475	8,527
Total available for sale, Gross Unrealized Losses	(64,585)	(73,206)
Total available for sale, Fair Value	4,239,561	4,187,040
Total fixed maturity securities available for sale	4,239,561	4,187,040
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,865,664	3,679,219
Available-for-sale, Allowance for Credit Loss	0	(40)
Total available for sale, Gross Unrealized Gains	14,741	24,312
Total available for sale, Gross Unrealized Losses	(146,130)	(143,936)
Total available for sale, Fair Value	3,734,275	3,559,555
Total fixed maturity securities available for sale	3,734,275	3,559,555
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,172,945	2,838,220
Available-for-sale, Allowance for Credit Loss	0	(4,986)
Total available for sale, Gross Unrealized Gains	10,100	14,681
Total available for sale, Gross Unrealized Losses	(69,749)	(68,681)
Total available for sale, Fair Value	3,113,296	2,779,234
Total fixed maturity securities available for sale	3,113,296	2,779,234
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	751,234	701,865
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,864	6,471
Total available for sale, Gross Unrealized Losses	(24,436)	(23,412)
Total available for sale, Fair Value	729,662	684,924
Total fixed maturity securities available for sale	729,662	684,924
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	590,644	635,975
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,516	1,605
Total available for sale, Gross Unrealized Losses	(7,690)	(7,234)
Total available for sale, Fair Value	585,470	630,346
Total fixed maturity securities available for sale	585,470	630,346
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	8,380,487	7,855,279
Available-for-sale, Allowance for Credit Loss	0	(5,026)	(3,795)	(4,701)
Total available for sale, Gross Unrealized Gains	30,221	47,069
Total available for sale, Gross Unrealized Losses	(248,005)	(243,263)
Total available for sale, Fair Value	8,162,703	7,654,059
Total fixed maturity securities available for sale	8,162,703	7,654,059
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,904,493	1,817,386
Available-for-sale, Allowance for Credit Loss	(20,479)	(29,603)	$ (33,324)	$ (32,633)
Total available for sale, Gross Unrealized Gains	8,585	15,865
Total available for sale, Gross Unrealized Losses	(174,347)	(137,419)
Total available for sale, Fair Value	1,718,252	1,666,229
Total fixed maturity securities available for sale	$ 1,718,252	$ 1,666,229

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