Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 73,388		$ 78,683
Held-to-maturity, Allowance for Credit Loss	(871)	$ 948	0
Held to maturity, Gross Unrealized Gains	14,515		13,994
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	87,032		92,677
Total held to maturity, Carrying Value	72,517		78,683
Total available for sale, Amortized Cost	13,462,876		13,897,964
Available-for-sale, Allowance for Credit Loss	(1,696)	(45,493)	0
Total available for sale, Gross Unrealized Gains	486,644		340,700
Total available for sale, Gross Unrealized Losses	(127,295)		(136,386)
Total available for sale, Fair Value	13,820,529		14,102,278
Total available for sale, Carrying Value	13,820,529		14,102,278
Total investments in fixed maturity securities, Amortized Value	13,536,264		13,976,647
Fixed maturity securities, allowance for credit loss	(2,567)
Total investments in fixed maturity securities, Gross Unrealized Gains	501,159		354,694
Total investments in fixed maturity securities, Gross Unrealized Losses	(127,295)		(136,386)
Total investments in fixed maturity securities, Fair Value	13,907,561		14,194,955
Total investments in fixed maturity securities, Carrying Value	13,893,046		14,180,961
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	66,532		70,312
Held-to-maturity, Allowance for Credit Loss	(871)
Held to maturity, Gross Unrealized Gains	13,406		13,000
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	79,067		83,312
Total held to maturity, Carrying Value	65,661		70,312
Total available for sale, Amortized Cost	3,430,081		3,754,641
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	199,001		146,918
Total available for sale, Gross Unrealized Losses	(3,687)		(5,927)
Total available for sale, Fair Value	3,625,395		3,895,632
Total available for sale, Carrying Value	3,625,395		3,895,632
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	6,856		8,371
Held-to-maturity, Allowance for Credit Loss	0
Held to maturity, Gross Unrealized Gains	1,109		994
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	7,965		9,365
Total held to maturity, Carrying Value	6,856		8,371
Total available for sale, Amortized Cost	848,878		1,298,145
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	29,989		23,230
Total available for sale, Gross Unrealized Losses	(2,513)		(5,155)
Total available for sale, Fair Value	876,354		1,316,220
Total available for sale, Carrying Value	876,354		1,316,220
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	676,493		775,157
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	21,080		13,249
Total available for sale, Gross Unrealized Losses	(141)		(1,475)
Total available for sale, Fair Value	697,432		786,931
Total available for sale, Carrying Value	697,432		786,931
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,195,851		2,343,209
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	93,278		64,586
Total available for sale, Gross Unrealized Losses	(2,455)		(4,152)
Total available for sale, Fair Value	2,286,674		2,403,643
Total available for sale, Carrying Value	2,286,674		2,403,643
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	356,321		359,298
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	31,586		22,074
Total available for sale, Gross Unrealized Losses	0		(97)
Total available for sale, Fair Value	387,907		381,275
Total available for sale, Carrying Value	387,907		381,275
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	257,570		364,571
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	22,521		20,342
Total available for sale, Gross Unrealized Losses	(149)		(128)
Total available for sale, Fair Value	279,942		384,785
Total available for sale, Carrying Value	279,942		384,785
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	223,508		255,230
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	9,641		7,232
Total available for sale, Gross Unrealized Losses	(549)		(903)
Total available for sale, Fair Value	232,600		261,559
Total available for sale, Carrying Value	232,600		261,559
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	396,831		432,333
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	41,975		32,684
Total available for sale, Gross Unrealized Losses	(534)		(647)
Total available for sale, Fair Value	438,272		464,370
Total available for sale, Carrying Value	438,272		464,370
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	199,217		304,506
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	7,168		5,214
Total available for sale, Gross Unrealized Losses	(360)		(346)
Total available for sale, Fair Value	206,025		309,374
Total available for sale, Carrying Value	206,025		309,374
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,048,095		1,602,651
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	37,157		28,444
Total available for sale, Gross Unrealized Losses	(2,873)		(5,501)
Total available for sale, Fair Value	1,082,379		1,625,594
Total available for sale, Carrying Value	1,082,379		1,625,594
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,347,867		2,802,588
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	11,665		9,532
Total available for sale, Gross Unrealized Losses	(53,093)		(21,490)
Total available for sale, Fair Value	3,306,439		2,790,630
Total available for sale, Carrying Value	3,306,439		2,790,630
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,240,914		2,260,073
Available-for-sale, Allowance for Credit Loss	(606)
Total available for sale, Gross Unrealized Gains	104,694		72,900
Total available for sale, Gross Unrealized Losses	(18,404)		(3,800)
Total available for sale, Fair Value	2,326,598		2,329,173
Total available for sale, Carrying Value	2,326,598		2,329,173
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,471,997		1,447,589
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	55,706		37,681
Total available for sale, Gross Unrealized Losses	(3,067)		(4,118)
Total available for sale, Fair Value	1,524,636		1,481,152
Total available for sale, Carrying Value	1,524,636		1,481,152
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	325,809		325,762
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	30,270		15,281
Total available for sale, Gross Unrealized Losses	(460)		(402)
Total available for sale, Fair Value	355,619		340,641
Total available for sale, Carrying Value	355,619		340,641
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	32,262		5,219
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	445		230
Total available for sale, Gross Unrealized Losses	(20)		0
Total available for sale, Fair Value	32,687		5,449
Total available for sale, Carrying Value	32,687		5,449
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,070,982		4,038,643
Available-for-sale, Allowance for Credit Loss	(606)	(724)	0
Total available for sale, Gross Unrealized Gains	191,115		126,092
Total available for sale, Gross Unrealized Losses	(21,951)		(8,320)
Total available for sale, Fair Value	4,239,540		4,156,415
Total available for sale, Carrying Value	4,239,540		4,156,415
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	889,358		924,284
Available-for-sale, Allowance for Credit Loss	(1,090)	$ (44,769)	0
Total available for sale, Gross Unrealized Gains	26,626		16,465
Total available for sale, Gross Unrealized Losses	(45,550)		(93,673)
Total available for sale, Fair Value	869,344		847,076
Total available for sale, Carrying Value	$ 869,344		$ 847,076