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Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 52,364		$ 51,410
Held-to-maturity, Allowance for Credit Loss	(53)	$ (107)	(114)	$ (127)	$ (378)	$ (387)
Held to maturity, Gross Unrealized Gains	2,882		4,277
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	55,193		55,573
Total held to maturity, Carrying Value	52,311		51,296
Total available for sale, Amortized Cost	19,051,984		18,664,073
Available-for-sale, Allowance for Credit Loss	(44,248)	(37,142)	(37,352)	(33,278)	(26,153)	(22,238)
Total available for sale, Gross Unrealized Gains	24,829		32,166
Total available for sale, Gross Unrealized Losses	(1,029,253)		(1,122,834)
Total available for sale, Fair Value	18,003,312		17,536,053
Total fixed maturity securities available for sale	18,003,312		17,536,053
Total investments in fixed maturity securities, Amortized Value	19,104,348		18,715,483
Fixed maturity securities, allowance for credit loss	(44,301)		(37,466)
Total investments in fixed maturity securities, Gross Unrealized Gains	27,711		36,443
Total investments in fixed maturity securities, Gross Unrealized Losses	(1,029,253)		(1,122,834)
Total investments in fixed maturity securities, Fair Value	18,058,505		17,591,626
Total investments in fixed maturity securities, Carrying Value	18,055,623		17,587,349
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	49,156		47,802
Held-to-maturity, Allowance for Credit Loss	(53)		(114)
Held to maturity, Gross Unrealized Gains	2,859		4,239
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	51,962		51,927
Total held to maturity, Carrying Value	49,103		47,688
Total available for sale, Amortized Cost	2,837,668		3,046,335
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	10,371		12,355
Total available for sale, Gross Unrealized Losses	(144,995)		(168,592)
Total available for sale, Fair Value	2,703,044		2,890,098
Total fixed maturity securities available for sale	2,703,044		2,890,098
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	3,208		3,608
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	23		38
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	3,231		3,646
Total held to maturity, Carrying Value	3,208		3,608
Total available for sale, Amortized Cost	1,470,034		1,308,019
Available-for-sale, Allowance for Credit Loss	(24)		(18)
Total available for sale, Gross Unrealized Gains	358		395
Total available for sale, Gross Unrealized Losses	(170,792)		(171,595)
Total available for sale, Fair Value	1,299,576		1,136,801
Total fixed maturity securities available for sale	1,299,576		1,136,801
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,306,926		960,479
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,917		937
Total available for sale, Gross Unrealized Losses	(70,726)		(69,158)
Total available for sale, Fair Value	1,238,117		892,258
Total fixed maturity securities available for sale	1,238,117		892,258
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,741,545		1,837,309
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	3,632		3,662
Total available for sale, Gross Unrealized Losses	(103,268)		(119,474)
Total available for sale, Fair Value	1,641,909		1,721,497
Total fixed maturity securities available for sale	1,641,909		1,721,497
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	369,064		387,709
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,042		2,651
Total available for sale, Gross Unrealized Losses	(17,958)		(21,335)
Total available for sale, Fair Value	353,148		369,025
Total fixed maturity securities available for sale	353,148		369,025
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	105,615		156,106
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,780		2,741
Total available for sale, Gross Unrealized Losses	(309)		(7)
Total available for sale, Fair Value	107,086		158,840
Total fixed maturity securities available for sale	107,086		158,840
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	197,728		210,228
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	290		334
Total available for sale, Gross Unrealized Losses	(9,277)		(10,923)
Total available for sale, Fair Value	188,741		199,639
Total fixed maturity securities available for sale	188,741		199,639
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	423,716		454,983
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,627		2,967
Total available for sale, Gross Unrealized Losses	(14,183)		(16,853)
Total available for sale, Fair Value	412,160		441,097
Total fixed maturity securities available for sale	412,160		441,097
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	624,077		547,757
Available-for-sale, Allowance for Credit Loss	(861)		0
Total available for sale, Gross Unrealized Gains	117		215
Total available for sale, Gross Unrealized Losses	(19,237)		(19,363)
Total available for sale, Fair Value	604,096		528,609
Total fixed maturity securities available for sale	604,096		528,609
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,094,111		1,855,776
Available-for-sale, Allowance for Credit Loss	(885)	(23)	(18)
Total available for sale, Gross Unrealized Gains	475		610
Total available for sale, Gross Unrealized Losses	(190,029)		(190,958)
Total available for sale, Fair Value	1,903,672		1,665,410
Total fixed maturity securities available for sale	1,903,672		1,665,410
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,861,009		4,132,365
Available-for-sale, Allowance for Credit Loss	(1,444)	0	0
Total available for sale, Gross Unrealized Gains	274		2,730
Total available for sale, Gross Unrealized Losses	(116,036)		(152,322)
Total available for sale, Fair Value	3,743,803		3,982,773
Total fixed maturity securities available for sale	3,743,803		3,982,773
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,502,256		3,491,645
Available-for-sale, Allowance for Credit Loss	0		(1,704)
Total available for sale, Gross Unrealized Gains	5,327		4,439
Total available for sale, Gross Unrealized Losses	(207,655)		(241,381)
Total available for sale, Fair Value	3,299,928		3,252,999
Total fixed maturity securities available for sale	3,299,928		3,252,999
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,749,952		2,585,247
Available-for-sale, Allowance for Credit Loss	(8,867)		(2,997)
Total available for sale, Gross Unrealized Gains	2,178		5,505
Total available for sale, Gross Unrealized Losses	(125,163)		(117,383)
Total available for sale, Fair Value	2,618,100		2,470,372
Total fixed maturity securities available for sale	2,618,100		2,470,372
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	654,356		586,066
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,555		1,307
Total available for sale, Gross Unrealized Losses	(34,486)		(36,325)
Total available for sale, Fair Value	621,425		551,048
Total fixed maturity securities available for sale	621,425		551,048
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	478,682		441,230
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	13		0
Total available for sale, Gross Unrealized Losses	(11,080)		(11,657)
Total available for sale, Fair Value	467,615		429,573
Total fixed maturity securities available for sale	467,615		429,573
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	7,385,246		7,104,188
Available-for-sale, Allowance for Credit Loss	(8,867)	(3,795)	(4,701)	(182)	0	(16)
Total available for sale, Gross Unrealized Gains	9,073		11,251
Total available for sale, Gross Unrealized Losses	(378,384)		(406,746)
Total available for sale, Fair Value	7,007,068		6,703,992
Total fixed maturity securities available for sale	7,007,068		6,703,992
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,567,024		1,564,930
Available-for-sale, Allowance for Credit Loss	(33,052)	$ (33,324)	(32,633)	$ (33,096)	$ (26,153)	$ (22,222)
Total available for sale, Gross Unrealized Gains	2,719		4,283
Total available for sale, Gross Unrealized Losses	(129,083)		(135,058)
Total available for sale, Fair Value	1,407,608		1,401,522
Total fixed maturity securities available for sale	$ 1,407,608		$ 1,401,522

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