Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 50,955		$ 74,368
Held-to-maturity, Allowance for Credit Loss	(121)	$ (127)	(387)	$ (396)	$ (453)	$ (798)
Held to maturity, Gross Unrealized Gains	3,550		11,445
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	54,384		85,426
Total held to maturity, Carrying Value	50,834		73,981
Total available for sale, Amortized Cost	18,416,706		16,396,936
Available-for-sale, Allowance for Credit Loss	(34,595)	(33,278)	(22,238)	(16,887)	(18,911)	(1,782)
Total available for sale, Gross Unrealized Gains	15,955		278,207
Total available for sale, Gross Unrealized Losses	(1,217,964)		(124,213)
Total available for sale, Fair Value	17,180,102		16,528,692
Total available for sale, Carrying Value	17,180,102		16,528,692
Total investments in fixed maturity securities, Amortized Value	18,467,661		16,471,304
Fixed maturity securities, allowance for credit loss	(34,716)		(22,625)
Total investments in fixed maturity securities, Gross Unrealized Gains	19,505		289,652
Total investments in fixed maturity securities, Gross Unrealized Losses	(1,217,964)		(124,213)
Total investments in fixed maturity securities, Fair Value	17,234,486		16,614,118
Total investments in fixed maturity securities, Carrying Value	17,230,936		16,602,673
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	47,139		69,539
Held-to-maturity, Allowance for Credit Loss	(121)		(387)
Held to maturity, Gross Unrealized Gains	3,522		10,813
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	50,540		79,965
Total held to maturity, Carrying Value	47,018		69,152
Total available for sale, Amortized Cost	3,049,021		3,176,042
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8,723		137,650
Total available for sale, Gross Unrealized Losses	(184,620)		(9,559)
Total available for sale, Fair Value	2,873,124		3,304,133
Total available for sale, Carrying Value	2,873,124		3,304,133
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	3,816		4,829
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	28		632
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	3,844		5,461
Total held to maturity, Carrying Value	3,816		4,829
Total available for sale, Amortized Cost	1,297,413		940,744
Available-for-sale, Allowance for Credit Loss	(21)		0
Total available for sale, Gross Unrealized Gains	210		9,896
Total available for sale, Gross Unrealized Losses	(163,514)		(11,321)
Total available for sale, Fair Value	1,134,088		939,319
Total available for sale, Carrying Value	1,134,088		939,319
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	937,214		851,128
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,185		8,509
Total available for sale, Gross Unrealized Losses	(68,807)		(4,294)
Total available for sale, Fair Value	870,592		855,343
Total available for sale, Carrying Value	870,592		855,343
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,877,116		2,016,382
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	3,235		62,961
Total available for sale, Gross Unrealized Losses	(128,427)		(5,706)
Total available for sale, Fair Value	1,751,924		2,073,637
Total available for sale, Carrying Value	1,751,924		2,073,637
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	366,596		388,110
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,907		23,152
Total available for sale, Gross Unrealized Losses	(22,956)		(1,015)
Total available for sale, Fair Value	345,547		410,247
Total available for sale, Carrying Value	345,547		410,247
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	156,685		202,633
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	946		14,891
Total available for sale, Gross Unrealized Losses	(200)		(574)
Total available for sale, Fair Value	157,431		216,950
Total available for sale, Carrying Value	157,431		216,950
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	198,649		166,943
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	0		7,191
Total available for sale, Gross Unrealized Losses	(13,558)		(1,532)
Total available for sale, Fair Value	185,091		172,602
Total available for sale, Carrying Value	185,091		172,602
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	449,975		401,974
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	2,635		29,455
Total available for sale, Gross Unrealized Losses	(19,479)		(732)
Total available for sale, Fair Value	433,131		430,697
Total available for sale, Carrying Value	433,131		430,697
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	474,049		125,709
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	52		3,388
Total available for sale, Gross Unrealized Losses	(17,164)		(341)
Total available for sale, Fair Value	456,937		128,756
Total available for sale, Carrying Value	456,937		128,756
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,771,462		1,066,453
Available-for-sale, Allowance for Credit Loss	(21)	0	0
Total available for sale, Gross Unrealized Gains	262		13,284
Total available for sale, Gross Unrealized Losses	(180,678)		(11,662)
Total available for sale, Fair Value	1,591,025		1,068,075
Total available for sale, Carrying Value	1,591,025		1,068,075
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,402,479		4,504,950
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	433		4,409
Total available for sale, Gross Unrealized Losses	(161,474)		(18,794)
Total available for sale, Fair Value	4,241,438		4,490,565
Total available for sale, Carrying Value	4,241,438		4,490,565
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,408,585		3,231,520
Available-for-sale, Allowance for Credit Loss	(319)		(16)
Total available for sale, Gross Unrealized Gains	1,604		62,751
Total available for sale, Gross Unrealized Losses	(278,084)		(21,092)
Total available for sale, Fair Value	3,131,786		3,273,163
Total available for sale, Carrying Value	3,131,786		3,273,163
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,411,487		1,739,282
Available-for-sale, Allowance for Credit Loss	(660)		0
Total available for sale, Gross Unrealized Gains	1,345		30,709
Total available for sale, Gross Unrealized Losses	(142,620)		(6,591)
Total available for sale, Fair Value	2,269,552		1,763,400
Total available for sale, Carrying Value	2,269,552		1,763,400
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	499,140		396,242
Available-for-sale, Allowance for Credit Loss	(220)		0
Total available for sale, Gross Unrealized Gains	0		13,262
Total available for sale, Gross Unrealized Losses	(40,823)		(3,202)
Total available for sale, Fair Value	458,097		406,302
Total available for sale, Carrying Value	458,097		406,302
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	357,472		154,210
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8		125
Total available for sale, Gross Unrealized Losses	(12,171)		(1,525)
Total available for sale, Fair Value	345,309		152,810
Total available for sale, Carrying Value	345,309		152,810
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	6,676,684		5,521,254
Available-for-sale, Allowance for Credit Loss	(1,199)	(182)	(16)	(12)	(12)	(518)
Total available for sale, Gross Unrealized Gains	2,957		106,847
Total available for sale, Gross Unrealized Losses	(473,698)		(32,410)
Total available for sale, Fair Value	6,204,744		5,595,675
Total available for sale, Carrying Value	6,204,744		5,595,675
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,579,846		1,277,109
Available-for-sale, Allowance for Credit Loss	(33,375)	$ (33,096)	(22,222)	$ (16,875)	$ (18,899)	$ (1,264)
Total available for sale, Gross Unrealized Gains	1,395		7,508
Total available for sale, Gross Unrealized Losses	(148,687)		(47,494)
Total available for sale, Fair Value	1,399,179		1,214,901
Total available for sale, Carrying Value	$ 1,399,179		$ 1,214,901