Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Held to maturity, Amortized cost	$ 43,959		$ 53,415
Held-to-maturity, Allowance for Credit Loss	(30)	$ (34)	(43)	$ (48)	$ (53)	$ (114)
Held to maturity, Gross Unrealized Gains	2,053		3,239
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	45,982		56,611
Held to maturity, Carrying Value	43,929		53,372
Total available for sale, Amortized Cost	22,959,189		20,861,830
Available-for-sale, Allowance for Credit Loss	(6,953)	(21,837)	(36,708)	(46,364)	(44,248)	(37,352)
Total available for sale, Gross Unrealized Gains	255,622		110,989
Total available for sale, Gross Unrealized Losses	(587,909)		(811,175)
Total available for sale, Fair Value	22,619,949		20,124,936
Total fixed maturity securities available for sale	22,619,949		20,124,936
Fixed maturity securities, amortized cost	23,003,148		20,915,245
Fixed maturity securities, allowance for credit loss	(6,983)		(36,751)
Total investments in fixed maturity securities, Gross Unrealized Gains	257,675		114,228
Total investments in fixed maturity securities, Gross Unrealized Losses	(587,909)		(811,175)
Total investments in fixed maturity securities, Fair Value	22,665,931		20,181,547
Total investments in fixed maturity securities, Carrying Value	22,663,878		20,178,308
State and municipal
Debt Securities, Available-for-sale [Line Items]
Held to maturity, Amortized cost	41,561		50,547
Held-to-maturity, Allowance for Credit Loss	(30)		(43)
Held to maturity, Gross Unrealized Gains	1,944		3,132
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	43,475		53,636
Held to maturity, Carrying Value	41,531		50,504
Total available for sale, Amortized Cost	2,567,941		2,742,922
Available-for-sale, Allowance for Credit Loss	0	(339)	(757)	(821)	0	0
Total available for sale, Gross Unrealized Gains	18,569		14,719
Total available for sale, Gross Unrealized Losses	(70,208)		(122,462)
Total available for sale, Fair Value	2,516,302		2,634,422
Total fixed maturity securities available for sale	2,516,302		2,634,422
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Held to maturity, Amortized cost	2,398		2,868
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	109		107
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	2,507		2,975
Held to maturity, Carrying Value	2,398		2,868
Total available for sale, Amortized Cost	2,736,493		1,773,206
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	31,527		12,780
Total available for sale, Gross Unrealized Losses	(130,929)		(163,844)
Total available for sale, Fair Value	2,637,091		1,622,142
Total fixed maturity securities available for sale	2,637,091		1,622,142
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,083,085		1,762,997
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	23,974		11,403
Total available for sale, Gross Unrealized Losses	(36,074)		(57,669)
Total available for sale, Fair Value	2,070,985		1,716,731
Total fixed maturity securities available for sale	2,070,985		1,716,731
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,732,996		1,682,550
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8,414		5,651
Total available for sale, Gross Unrealized Losses	(51,652)		(82,006)
Total available for sale, Fair Value	1,689,758		1,606,195
Total fixed maturity securities available for sale	1,689,758		1,606,195
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	273,232		394,429
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	4,565		3,550
Total available for sale, Gross Unrealized Losses	(7,375)		(16,405)
Total available for sale, Fair Value	270,422		381,574
Total fixed maturity securities available for sale	270,422		381,574
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	85,683		103,029
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,260		1,634
Total available for sale, Gross Unrealized Losses	(90)		(185)
Total available for sale, Fair Value	86,853		104,478
Total fixed maturity securities available for sale	86,853		104,478
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	168,276		166,873
Available-for-sale, Allowance for Credit Loss	0		(757)
Total available for sale, Gross Unrealized Gains	1,847		696
Total available for sale, Gross Unrealized Losses	(4,968)		(11,973)
Total available for sale, Fair Value	165,155		154,839
Total fixed maturity securities available for sale	165,155		154,839
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	307,754		396,041
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,483		3,188
Total available for sale, Gross Unrealized Losses	(6,123)		(11,893)
Total available for sale, Fair Value	304,114		387,336
Total fixed maturity securities available for sale	304,114		387,336
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	600,597		657,157
Available-for-sale, Allowance for Credit Loss	(52)		(158)
Total available for sale, Gross Unrealized Gains	3,254		626
Total available for sale, Gross Unrealized Losses	(2,918)		(13,312)
Total available for sale, Fair Value	600,881		644,313
Total fixed maturity securities available for sale	600,881		644,313
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,337,090		2,430,363
Available-for-sale, Allowance for Credit Loss	(52)	(1,140)	(158)	(2,100)	(885)	(18)
Total available for sale, Gross Unrealized Gains	34,781		13,406
Total available for sale, Gross Unrealized Losses	(133,847)		(177,156)
Total available for sale, Fair Value	3,237,972		2,266,455
Total fixed maturity securities available for sale	3,237,972		2,266,455
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,118,052		4,252,883
Available-for-sale, Allowance for Credit Loss	(546)	(889)	(1,164)	(1,502)	(1,444)	0
Total available for sale, Gross Unrealized Gains	26,592		8,527
Total available for sale, Gross Unrealized Losses	(36,777)		(73,206)
Total available for sale, Fair Value	4,107,321		4,187,040
Total fixed maturity securities available for sale	4,107,321		4,187,040
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,980,058		3,679,219
Available-for-sale, Allowance for Credit Loss	0		(40)
Total available for sale, Gross Unrealized Gains	55,042		24,312
Total available for sale, Gross Unrealized Losses	(80,692)		(143,936)
Total available for sale, Fair Value	3,954,408		3,559,555
Total fixed maturity securities available for sale	3,954,408		3,559,555
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,326,838		2,838,220
Available-for-sale, Allowance for Credit Loss	0		(4,986)
Total available for sale, Gross Unrealized Gains	54,929		14,681
Total available for sale, Gross Unrealized Losses	(33,205)		(68,681)
Total available for sale, Fair Value	3,348,562		2,779,234
Total fixed maturity securities available for sale	3,348,562		2,779,234
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	774,956		701,865
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	13,319		6,471
Total available for sale, Gross Unrealized Losses	(13,877)		(23,412)
Total available for sale, Fair Value	774,398		684,924
Total fixed maturity securities available for sale	774,398		684,924
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	774,274		635,975
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	4,715		1,605
Total available for sale, Gross Unrealized Losses	(3,593)		(7,234)
Total available for sale, Fair Value	775,396		630,346
Total fixed maturity securities available for sale	775,396		630,346
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	8,856,126		7,855,279
Available-for-sale, Allowance for Credit Loss	0	0	(5,026)	(5,770)	(8,867)	(4,701)
Total available for sale, Gross Unrealized Gains	128,005		47,069
Total available for sale, Gross Unrealized Losses	(131,367)		(243,263)
Total available for sale, Fair Value	8,852,764		7,654,059
Total fixed maturity securities available for sale	8,852,764		7,654,059
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,996,895		1,817,386
Available-for-sale, Allowance for Credit Loss	(6,355)	$ (19,469)	(29,603)	$ (36,171)	$ (33,052)	$ (32,633)
Total available for sale, Gross Unrealized Gains	23,701		15,865
Total available for sale, Gross Unrealized Losses	(179,636)		(137,419)
Total available for sale, Fair Value	1,834,605		1,666,229
Total fixed maturity securities available for sale	$ 1,834,605		$ 1,666,229