Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 79,025		$ 78,683
Held-to-maturity, Allowance for Credit Loss	(948)	$ 107	0
Held to maturity, Gross Unrealized Gains	15,313		13,994
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	93,390		92,677
Total held to maturity, Carrying Value	78,077		78,683
Total available for sale, Amortized Cost	12,940,353		13,897,964
Available-for-sale, Allowance for Credit Loss	(45,493)	(67,356)	0
Total available for sale, Gross Unrealized Gains	471,214		340,700
Total available for sale, Gross Unrealized Losses	(163,066)		(136,386)
Total available for sale, Fair Value	13,203,008		14,102,278
Total available for sale, Carrying Value	13,203,008		14,102,278
Total investments in fixed maturity securities, Amortized Value	13,019,378		13,976,647
Fixed maturity securities, allowance for credit loss	(46,441)
Total investments in fixed maturity securities, Gross Unrealized Gains	486,527		354,694
Total investments in fixed maturity securities, Gross Unrealized Losses	(163,066)		(136,386)
Total investments in fixed maturity securities, Fair Value	13,296,398		14,194,955
Total investments in fixed maturity securities, Carrying Value	13,281,085		14,180,961
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	71,640		70,312
Held-to-maturity, Allowance for Credit Loss	(948)
Held to maturity, Gross Unrealized Gains	14,137		13,000
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	84,829		83,312
Total held to maturity, Carrying Value	70,692		70,312
Total available for sale, Amortized Cost	3,422,224		3,754,641
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	185,713		146,918
Total available for sale, Gross Unrealized Losses	(3,242)		(5,927)
Total available for sale, Fair Value	3,604,695		3,895,632
Total available for sale, Carrying Value	3,604,695		3,895,632
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	7,385		8,371
Held-to-maturity, Allowance for Credit Loss	0
Held to maturity, Gross Unrealized Gains	1,176		994
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	8,561		9,365
Total held to maturity, Carrying Value	7,385		8,371
Total available for sale, Amortized Cost	913,065		1,298,145
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	32,241		23,230
Total available for sale, Gross Unrealized Losses	(1,019)		(5,155)
Total available for sale, Fair Value	944,287		1,316,220
Total available for sale, Carrying Value	944,287		1,316,220
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	670,032		775,157
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	23,784		13,249
Total available for sale, Gross Unrealized Losses	(49)		(1,475)
Total available for sale, Fair Value	693,767		786,931
Total available for sale, Carrying Value	693,767		786,931
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,153,909		2,343,209
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	82,603		64,586
Total available for sale, Gross Unrealized Losses	(1,551)		(4,152)
Total available for sale, Fair Value	2,234,961		2,403,643
Total available for sale, Carrying Value	2,234,961		2,403,643
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	350,228		359,298
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	31,192		22,074
Total available for sale, Gross Unrealized Losses	(810)		(97)
Total available for sale, Fair Value	380,610		381,275
Total available for sale, Carrying Value	380,610		381,275
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	299,333		364,571
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	20,944		20,342
Total available for sale, Gross Unrealized Losses	(130)		(128)
Total available for sale, Fair Value	320,147		384,785
Total available for sale, Carrying Value	320,147		384,785
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	224,649		255,230
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	8,581		7,232
Total available for sale, Gross Unrealized Losses	(500)		(903)
Total available for sale, Fair Value	232,730		261,559
Total available for sale, Carrying Value	232,730		261,559
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	394,105		432,333
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	42,393		32,684
Total available for sale, Gross Unrealized Losses	(251)		(647)
Total available for sale, Fair Value	436,247		464,370
Total available for sale, Carrying Value	436,247		464,370
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	211,987		304,506
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	6,370		5,214
Total available for sale, Gross Unrealized Losses	(799)		(346)
Total available for sale, Fair Value	217,558		309,374
Total available for sale, Carrying Value	217,558		309,374
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,125,052		1,602,651
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	38,611		28,444
Total available for sale, Gross Unrealized Losses	(1,818)		(5,501)
Total available for sale, Fair Value	1,161,845		1,625,594
Total available for sale, Carrying Value	1,161,845		1,625,594
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,170,931		2,802,588
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	10,539		9,532
Total available for sale, Gross Unrealized Losses	(83,272)		(21,490)
Total available for sale, Fair Value	3,098,198		2,790,630
Total available for sale, Carrying Value	3,098,198		2,790,630
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,922,336		2,260,073
Available-for-sale, Allowance for Credit Loss	(724)
Total available for sale, Gross Unrealized Gains	101,976		72,900
Total available for sale, Gross Unrealized Losses	(24,466)		(3,800)
Total available for sale, Fair Value	1,999,122		2,329,173
Total available for sale, Carrying Value	1,999,122		2,329,173
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,399,238		1,447,589
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	49,596		37,681
Total available for sale, Gross Unrealized Losses	(17,130)		(4,118)
Total available for sale, Fair Value	1,431,704		1,481,152
Total available for sale, Carrying Value	1,431,704		1,481,152
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	293,279		325,762
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	29,582		15,281
Total available for sale, Gross Unrealized Losses	(146)		(402)
Total available for sale, Fair Value	322,715		340,641
Total available for sale, Carrying Value	322,715		340,641
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	18,655		5,219
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	385		230
Total available for sale, Gross Unrealized Losses	0		0
Total available for sale, Fair Value	19,040		5,449
Total available for sale, Carrying Value	19,040		5,449
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,633,508		4,038,643
Available-for-sale, Allowance for Credit Loss	(724)	(6,436)	0
Total available for sale, Gross Unrealized Gains	181,539		126,092
Total available for sale, Gross Unrealized Losses	(41,742)		(8,320)
Total available for sale, Fair Value	3,772,581		4,156,415
Total available for sale, Carrying Value	3,772,581		4,156,415
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	918,606		924,284
Available-for-sale, Allowance for Credit Loss	(44,769)	$ (60,920)	0
Total available for sale, Gross Unrealized Gains	31,028		16,465
Total available for sale, Gross Unrealized Losses	(32,943)		(93,673)
Total available for sale, Fair Value	871,922		847,076
Total available for sale, Carrying Value	$ 871,922		$ 847,076