Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 52,876		$ 51,410
Held-to-maturity, Allowance for Credit Loss	(48)	$ (53)	(114)	$ (121)	$ (127)	$ (387)
Held to maturity, Gross Unrealized Gains	1,844		4,277
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	54,672		55,573
Total held to maturity, Carrying Value	52,828		51,296
Total available for sale, Amortized Cost	20,179,897		18,664,073
Available-for-sale, Allowance for Credit Loss	(46,364)	(44,248)	(37,352)	(34,595)	(33,278)	(22,238)
Total available for sale, Gross Unrealized Gains	16,946		32,166
Total available for sale, Gross Unrealized Losses	(1,168,002)		(1,122,834)
Total available for sale, Fair Value	18,982,477		17,536,053
Total fixed maturity securities available for sale	18,982,477		17,536,053
Total investments in fixed maturity securities, Amortized Value	20,232,773		18,715,483
Fixed maturity securities, allowance for credit loss	(46,412)		(37,466)
Total investments in fixed maturity securities, Gross Unrealized Gains	18,790		36,443
Total investments in fixed maturity securities, Gross Unrealized Losses	(1,168,002)		(1,122,834)
Total investments in fixed maturity securities, Fair Value	19,037,149		17,591,626
Total investments in fixed maturity securities, Carrying Value	19,035,305		17,587,349
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	49,847		47,802
Held-to-maturity, Allowance for Credit Loss	(48)		(114)
Held to maturity, Gross Unrealized Gains	1,825		4,239
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	51,624		51,927
Total held to maturity, Carrying Value	49,799		47,688
Total available for sale, Amortized Cost	2,849,350		3,046,335
Available-for-sale, Allowance for Credit Loss	(821)	0	0
Total available for sale, Gross Unrealized Gains	5,662		12,355
Total available for sale, Gross Unrealized Losses	(183,193)		(168,592)
Total available for sale, Fair Value	2,670,998		2,890,098
Total fixed maturity securities available for sale	2,670,998		2,890,098
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	3,029		3,608
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	19		38
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	3,048		3,646
Total held to maturity, Carrying Value	3,029		3,608
Total available for sale, Amortized Cost	1,660,790		1,308,019
Available-for-sale, Allowance for Credit Loss	(24)		(18)
Total available for sale, Gross Unrealized Gains	399		395
Total available for sale, Gross Unrealized Losses	(214,381)		(171,595)
Total available for sale, Fair Value	1,446,784		1,136,801
Total fixed maturity securities available for sale	1,446,784		1,136,801
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,546,889		960,479
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,077		937
Total available for sale, Gross Unrealized Losses	(81,767)		(69,158)
Total available for sale, Fair Value	1,467,199		892,258
Total fixed maturity securities available for sale	1,467,199		892,258
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,771,219		1,837,309
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,287		3,662
Total available for sale, Gross Unrealized Losses	(126,585)		(119,474)
Total available for sale, Fair Value	1,646,921		1,721,497
Total fixed maturity securities available for sale	1,646,921		1,721,497
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	378,650		387,709
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,247		2,651
Total available for sale, Gross Unrealized Losses	(23,727)		(21,335)
Total available for sale, Fair Value	356,170		369,025
Total fixed maturity securities available for sale	356,170		369,025
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	103,219		156,106
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	202		2,741
Total available for sale, Gross Unrealized Losses	(827)		(7)
Total available for sale, Fair Value	102,594		158,840
Total fixed maturity securities available for sale	102,594		158,840
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	194,976		210,228
Available-for-sale, Allowance for Credit Loss	(821)		0
Total available for sale, Gross Unrealized Gains	159		334
Total available for sale, Gross Unrealized Losses	(14,341)		(10,923)
Total available for sale, Fair Value	179,973		199,639
Total fixed maturity securities available for sale	179,973		199,639
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	401,286		454,983
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,767		2,967
Total available for sale, Gross Unrealized Losses	(17,713)		(16,853)
Total available for sale, Fair Value	385,340		441,097
Total fixed maturity securities available for sale	385,340		441,097
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	651,966		547,757
Available-for-sale, Allowance for Credit Loss	(2,076)		0
Total available for sale, Gross Unrealized Gains	11		215
Total available for sale, Gross Unrealized Losses	(20,389)		(19,363)
Total available for sale, Fair Value	629,512		528,609
Total fixed maturity securities available for sale	629,512		528,609
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,312,756		1,855,776
Available-for-sale, Allowance for Credit Loss	(2,100)	(885)	(18)	(21)	0	0
Total available for sale, Gross Unrealized Gains	410		610
Total available for sale, Gross Unrealized Losses	(234,770)		(190,958)
Total available for sale, Fair Value	2,076,296		1,665,410
Total fixed maturity securities available for sale	2,076,296		1,665,410
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,197,235		4,132,365
Available-for-sale, Allowance for Credit Loss	(1,502)	(1,444)	0
Total available for sale, Gross Unrealized Gains	750		2,730
Total available for sale, Gross Unrealized Losses	(109,025)		(152,322)
Total available for sale, Fair Value	4,087,458		3,982,773
Total fixed maturity securities available for sale	4,087,458		3,982,773
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,567,020		3,491,645
Available-for-sale, Allowance for Credit Loss	0		(1,704)
Total available for sale, Gross Unrealized Gains	5,026		4,439
Total available for sale, Gross Unrealized Losses	(227,479)		(241,381)
Total available for sale, Fair Value	3,344,567		3,252,999
Total fixed maturity securities available for sale	3,344,567		3,252,999
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,813,603		2,585,247
Available-for-sale, Allowance for Credit Loss	(5,406)		(2,997)
Total available for sale, Gross Unrealized Gains	1,362		5,505
Total available for sale, Gross Unrealized Losses	(130,312)		(117,383)
Total available for sale, Fair Value	2,679,247		2,470,372
Total fixed maturity securities available for sale	2,679,247		2,470,372
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	683,468		586,066
Available-for-sale, Allowance for Credit Loss	(364)		0
Total available for sale, Gross Unrealized Gains	164		1,307
Total available for sale, Gross Unrealized Losses	(39,991)		(36,325)
Total available for sale, Fair Value	643,277		551,048
Total fixed maturity securities available for sale	643,277		551,048
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	593,466		441,230
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	134		0
Total available for sale, Gross Unrealized Losses	(11,877)		(11,657)
Total available for sale, Fair Value	581,723		429,573
Total fixed maturity securities available for sale	581,723		429,573
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	7,657,557		7,104,188
Available-for-sale, Allowance for Credit Loss	(5,770)	(8,867)	(4,701)	(1,199)	(182)	(16)
Total available for sale, Gross Unrealized Gains	6,686		11,251
Total available for sale, Gross Unrealized Losses	(409,659)		(406,746)
Total available for sale, Fair Value	7,248,814		6,703,992
Total fixed maturity securities available for sale	7,248,814		6,703,992
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,616,110		1,564,930
Available-for-sale, Allowance for Credit Loss	(36,171)	$ (33,052)	(32,633)	$ (33,375)	$ (33,096)	$ (22,222)
Total available for sale, Gross Unrealized Gains	1,361		4,283
Total available for sale, Gross Unrealized Losses	(149,588)		(135,058)
Total available for sale, Fair Value	1,431,712		1,401,522
Total fixed maturity securities available for sale	$ 1,431,712		$ 1,401,522