Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 78,859	$ 78,683
Held-to-maturity, Allowance for Credit Loss	(107)	0
Held to maturity, Gross Unrealized Gains	12,324	13,994
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	91,076	92,677
Total held to maturity, Carrying Value	78,752	78,683
Total available for sale, Amortized Cost	13,739,588	13,897,964
Available-for-sale, Allowance for Credit Loss	(67,356)	0
Total available for sale, Gross Unrealized Gains	276,460	340,700
Total available for sale, Gross Unrealized Losses	(361,377)	(136,386)
Total available for sale, Fair Value	13,587,315	14,102,278
Total available for sale, Carrying Value	13,587,315	14,102,278
Total investments in fixed maturity securities, Amortized Value	13,818,447	13,976,647
Fixed maturity securities, allowance for credit loss	(67,463)
Total investments in fixed maturity securities, Gross Unrealized Gains	288,784	354,694
Total investments in fixed maturity securities, Gross Unrealized Losses	(361,377)	(136,386)
Total investments in fixed maturity securities, Fair Value	13,678,391	14,194,955
Total investments in fixed maturity securities, Carrying Value	13,666,067	14,180,961
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	70,971	70,312
Held-to-maturity, Allowance for Credit Loss	(107)
Held to maturity, Gross Unrealized Gains	11,516	13,000
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	82,380	83,312
Total held to maturity, Carrying Value	70,864	70,312
Total available for sale, Amortized Cost	3,627,019	3,754,641
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	134,115	146,918
Total available for sale, Gross Unrealized Losses	(12,979)	(5,927)
Total available for sale, Fair Value	3,748,155	3,895,632
Total available for sale, Carrying Value	3,748,155	3,895,632
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	7,888	8,371
Held-to-maturity, Allowance for Credit Loss	0
Held to maturity, Gross Unrealized Gains	808	994
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	8,696	9,365
Total held to maturity, Carrying Value	7,888	8,371
Total available for sale, Amortized Cost	1,169,144	1,298,145
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	26,039	23,230
Total available for sale, Gross Unrealized Losses	(15,480)	(5,155)
Total available for sale, Fair Value	1,179,703	1,316,220
Total available for sale, Carrying Value	1,179,703	1,316,220
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	715,890	775,157
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	23,830	13,249
Total available for sale, Gross Unrealized Losses	(58)	(1,475)
Total available for sale, Fair Value	739,662	786,931
Total available for sale, Carrying Value	739,662	786,931
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,283,655	2,343,209
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	53,529	64,586
Total available for sale, Gross Unrealized Losses	(9,043)	(4,152)
Total available for sale, Fair Value	2,328,141	2,403,643
Total available for sale, Carrying Value	2,328,141	2,403,643
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	365,109	359,298
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	22,440	22,074
Total available for sale, Gross Unrealized Losses	(1,592)	(97)
Total available for sale, Fair Value	385,957	381,275
Total available for sale, Carrying Value	385,957	381,275
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	311,077	364,571
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	18,281	20,342
Total available for sale, Gross Unrealized Losses	(128)	(128)
Total available for sale, Fair Value	329,230	384,785
Total available for sale, Carrying Value	329,230	384,785
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	254,887	255,230
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	4,880	7,232
Total available for sale, Gross Unrealized Losses	(1,705)	(903)
Total available for sale, Fair Value	258,062	261,559
Total available for sale, Carrying Value	258,062	261,559
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	412,291	432,333
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	34,985	32,684
Total available for sale, Gross Unrealized Losses	(511)	(647)
Total available for sale, Fair Value	446,765	464,370
Total available for sale, Carrying Value	446,765	464,370
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	284,584	304,506
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	841	5,214
Total available for sale, Gross Unrealized Losses	(9,688)	(346)
Total available for sale, Fair Value	275,737	309,374
Total available for sale, Carrying Value	275,737	309,374
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,453,728	1,602,651
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	26,880	28,444
Total available for sale, Gross Unrealized Losses	(25,168)	(5,501)
Total available for sale, Fair Value	1,455,440	1,625,594
Total available for sale, Carrying Value	1,455,440	1,625,594
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,123,627	2,802,588
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	2,415	9,532
Total available for sale, Gross Unrealized Losses	(187,990)	(21,490)
Total available for sale, Fair Value	2,938,052	2,790,630
Total available for sale, Carrying Value	2,938,052	2,790,630
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,175,423	2,260,073
Available-for-sale, Allowance for Credit Loss	(3,513)
Total available for sale, Gross Unrealized Gains	42,215	72,900
Total available for sale, Gross Unrealized Losses	(62,365)	(3,800)
Total available for sale, Fair Value	2,151,760	2,329,173
Total available for sale, Carrying Value	2,151,760	2,329,173
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,482,371	1,447,589
Available-for-sale, Allowance for Credit Loss	(2,923)
Total available for sale, Gross Unrealized Gains	16,083	37,681
Total available for sale, Gross Unrealized Losses	(28,128)	(4,118)
Total available for sale, Fair Value	1,467,403	1,481,152
Total available for sale, Carrying Value	1,467,403	1,481,152
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	327,101	325,762
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	9,315	15,281
Total available for sale, Gross Unrealized Losses	(1,838)	(402)
Total available for sale, Fair Value	334,578	340,641
Total available for sale, Carrying Value	334,578	340,641
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	24,560	5,219
Available-for-sale, Allowance for Credit Loss	0
Total available for sale, Gross Unrealized Gains	230	230
Total available for sale, Gross Unrealized Losses	(14)	0
Total available for sale, Fair Value	24,776	5,449
Total available for sale, Carrying Value	24,776	5,449
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,009,455	4,038,643
Available-for-sale, Allowance for Credit Loss	(6,436)	0
Total available for sale, Gross Unrealized Gains	67,843	126,092
Total available for sale, Gross Unrealized Losses	(92,345)	(8,320)
Total available for sale, Fair Value	3,978,517	4,156,415
Total available for sale, Carrying Value	3,978,517	4,156,415
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	809,869	924,284
Available-for-sale, Allowance for Credit Loss	(60,920)	0
Total available for sale, Gross Unrealized Gains	21,377	16,465
Total available for sale, Gross Unrealized Losses	(42,837)	(93,673)
Total available for sale, Fair Value	727,489	847,076
Total available for sale, Carrying Value	$ 727,489	$ 847,076