Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 50,641		$ 74,368
Held-to-maturity, Allowance for Credit Loss	(127)	$ (378)	(387)	$ (453)	$ (730)	$ (798)
Held to maturity, Gross Unrealized Gains	5,517		11,445
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	56,031		85,426
Total held to maturity, Carrying Value	50,514		73,981
Total available for sale, Amortized Cost	17,646,779		16,396,936
Available-for-sale, Allowance for Credit Loss	(33,278)	(26,153)	(22,238)	(18,911)	(20,009)	(1,782)
Total available for sale, Gross Unrealized Gains	27,704		278,207
Total available for sale, Gross Unrealized Losses	(840,205)		(124,213)
Total available for sale, Fair Value	16,801,000		16,528,692
Total available for sale, Carrying Value	16,801,000		16,528,692
Total investments in fixed maturity securities, Amortized Value	17,697,420		16,471,304
Fixed maturity securities, allowance for credit loss	(33,405)		(22,625)
Total investments in fixed maturity securities, Gross Unrealized Gains	33,221		289,652
Total investments in fixed maturity securities, Gross Unrealized Losses	(840,205)		(124,213)
Total investments in fixed maturity securities, Fair Value	16,857,031		16,614,118
Total investments in fixed maturity securities, Carrying Value	16,851,514		16,602,673
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	46,486		69,539
Held-to-maturity, Allowance for Credit Loss	(127)		(387)
Held to maturity, Gross Unrealized Gains	5,347		10,813
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	51,706		79,965
Total held to maturity, Carrying Value	46,359		69,152
Total available for sale, Amortized Cost	3,101,679		3,176,042
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	20,871		137,650
Total available for sale, Gross Unrealized Losses	(105,287)		(9,559)
Total available for sale, Fair Value	3,017,263		3,304,133
Total available for sale, Carrying Value	3,017,263		3,304,133
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	4,155		4,829
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	170		632
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	4,325		5,461
Total held to maturity, Carrying Value	4,155		4,829
Total available for sale, Amortized Cost	1,181,482		940,744
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	794		9,896
Total available for sale, Gross Unrealized Losses	(110,774)		(11,321)
Total available for sale, Fair Value	1,071,502		939,319
Total available for sale, Carrying Value	1,071,502		939,319
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	762,615		851,128
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,213		8,509
Total available for sale, Gross Unrealized Losses	(42,914)		(4,294)
Total available for sale, Fair Value	720,914		855,343
Total available for sale, Carrying Value	720,914		855,343
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,937,329		2,016,382
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	6,636		62,961
Total available for sale, Gross Unrealized Losses	(76,329)		(5,706)
Total available for sale, Fair Value	1,867,636		2,073,637
Total available for sale, Carrying Value	1,867,636		2,073,637
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	378,081		388,110
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	3,957		23,152
Total available for sale, Gross Unrealized Losses	(13,458)		(1,015)
Total available for sale, Fair Value	368,580		410,247
Total available for sale, Carrying Value	368,580		410,247
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	200,716		202,633
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	3,232		14,891
Total available for sale, Gross Unrealized Losses	(49)		(574)
Total available for sale, Fair Value	203,899		216,950
Total available for sale, Carrying Value	203,899		216,950
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	166,591		166,943
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	453		7,191
Total available for sale, Gross Unrealized Losses	(6,836)		(1,532)
Total available for sale, Fair Value	160,208		172,602
Total available for sale, Carrying Value	160,208		172,602
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	418,962		401,974
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	6,593		29,455
Total available for sale, Gross Unrealized Losses	(8,615)		(732)
Total available for sale, Fair Value	416,940		430,697
Total available for sale, Carrying Value	416,940		430,697
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	413,090		125,709
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	552		3,388
Total available for sale, Gross Unrealized Losses	(8,045)		(341)
Total available for sale, Fair Value	405,597		128,756
Total available for sale, Carrying Value	405,597		128,756
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,594,572		1,066,453
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	1,346		13,284
Total available for sale, Gross Unrealized Losses	(118,819)		(11,662)
Total available for sale, Fair Value	1,477,099		1,068,075
Total available for sale, Carrying Value	1,477,099		1,068,075
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,421,553		4,504,950
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	484		4,409
Total available for sale, Gross Unrealized Losses	(129,549)		(18,794)
Total available for sale, Fair Value	4,292,488		4,490,565
Total available for sale, Carrying Value	4,292,488		4,490,565
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,412,383		3,231,520
Available-for-sale, Allowance for Credit Loss	(164)		(16)
Total available for sale, Gross Unrealized Gains	2,643		62,751
Total available for sale, Gross Unrealized Losses	(199,225)		(21,092)
Total available for sale, Fair Value	3,215,637		3,273,163
Total available for sale, Carrying Value	3,215,637		3,273,163
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,909,682		1,739,282
Available-for-sale, Allowance for Credit Loss	(18)		0
Total available for sale, Gross Unrealized Gains	341		30,709
Total available for sale, Gross Unrealized Losses	(98,344)		(6,591)
Total available for sale, Fair Value	1,811,661		1,763,400
Total available for sale, Carrying Value	1,811,661		1,763,400
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	448,342		396,242
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	249		13,262
Total available for sale, Gross Unrealized Losses	(26,490)		(3,202)
Total available for sale, Fair Value	422,101		406,302
Total available for sale, Carrying Value	422,101		406,302
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	247,531		154,210
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	29		125
Total available for sale, Gross Unrealized Losses	(7,834)		(1,525)
Total available for sale, Fair Value	239,726		152,810
Total available for sale, Carrying Value	239,726		152,810
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	6,017,938		5,521,254
Available-for-sale, Allowance for Credit Loss	(182)	0	(16)	(12)	(16)	(518)
Total available for sale, Gross Unrealized Gains	3,262		106,847
Total available for sale, Gross Unrealized Losses	(331,893)		(32,410)
Total available for sale, Fair Value	5,689,125		5,595,675
Total available for sale, Carrying Value	5,689,125		5,595,675
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,748,422		1,277,109
Available-for-sale, Allowance for Credit Loss	(33,096)	$ (26,153)	(22,222)	$ (18,899)	$ (19,993)	$ (1,264)
Total available for sale, Gross Unrealized Gains	528		7,508
Total available for sale, Gross Unrealized Losses	(111,743)		(47,494)
Total available for sale, Fair Value	1,604,111		1,214,901
Total available for sale, Carrying Value	$ 1,604,111		$ 1,214,901