Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 78,683	$ 78,635
Total held to maturity, Gross Unrealized Gains	13,994	12,808
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	92,677	91,443
Total held to maturity, Carrying Value	78,683	78,635
Total available for sale, Amortized Cost	13,897,964	13,596,057
Total available for sale, Gross Unrealized Gains	340,700	147,683
Total available for sale, Gross Unrealized Losses	(136,386)	(215,563)
Total available for sale, Fair Value	14,102,278	13,528,177
Total available for sale, Carrying Value	14,102,278	13,528,177
Total investment in fixed maturity securities, Amortized Cost	13,976,647	13,674,692
Total investments in fixed maturity securities, Gross Unrealized Gains	354,694	160,491
Total investments in fixed maturity securities, Gross Unrealized Losses	(136,386)	(215,563)
Total investments in fixed maturity securities, Fair Value	14,194,955	13,619,620
Total investments in fixed maturity securities, carrying value	14,180,961	13,606,812
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	70,312	67,891
Total held to maturity, Gross Unrealized Gains	13,000	11,549
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	83,312	79,440
Total held to maturity, Carrying Value	70,312	67,891
Total available for sale, Amortized Cost	3,754,641	3,815,515
Total available for sale, Gross Unrealized Gains	146,918	81,695
Total available for sale, Gross Unrealized Losses	(5,927)	(24,612)
Total available for sale, Fair Value	3,895,632	3,872,598
Total available for sale, Carrying Value	3,895,632	3,872,598
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	8,371	10,744
Total held to maturity, Gross Unrealized Gains	994	1,259
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	9,365	12,003
Total held to maturity, Carrying Value	8,371	10,744
Total available for sale, Amortized Cost	1,298,145	1,264,376
Total available for sale, Gross Unrealized Gains	23,230	7,729
Total available for sale, Gross Unrealized Losses	(5,155)	(20,225)
Total available for sale, Fair Value	1,316,220	1,251,880
Total available for sale, Carrying Value	1,316,220	1,251,880
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	4,038,643	4,188,769
Total available for sale, Gross Unrealized Gains	126,092	25,852
Total available for sale, Gross Unrealized Losses	(8,320)	(103,310)
Total available for sale, Fair Value	4,156,415	4,111,311
Total available for sale, Carrying Value	4,156,415	4,111,311
U.S. government and government agency
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	775,157	697,931
Total available for sale, Gross Unrealized Gains	13,249	9,219
Total available for sale, Gross Unrealized Losses	(1,475)	(4,910)
Total available for sale, Fair Value	786,931	702,240
Total available for sale, Carrying Value	786,931	702,240
Special Revenue
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,343,209	2,396,089
Total available for sale, Gross Unrealized Gains	64,586	30,507
Total available for sale, Gross Unrealized Losses	(4,152)	(19,790)
Total available for sale, Fair Value	2,403,643	2,406,806
Total available for sale, Carrying Value	2,403,643	2,406,806
Debt, State Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	359,298	335,626
Total available for sale, Gross Unrealized Gains	22,074	11,951
Total available for sale, Gross Unrealized Losses	(97)	(1,103)
Total available for sale, Fair Value	381,275	346,474
Total available for sale, Carrying Value	381,275	346,474
Revenue Subject to Refund
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	364,571	408,141
Total available for sale, Gross Unrealized Gains	20,342	16,568
Total available for sale, Gross Unrealized Losses	(128)	(30)
Total available for sale, Fair Value	384,785	424,679
Total available for sale, Carrying Value	384,785	424,679
Corporation
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	255,230	272,440
Total available for sale, Gross Unrealized Gains	7,232	4,319
Total available for sale, Gross Unrealized Losses	(903)	(2,350)
Total available for sale, Fair Value	261,559	274,409
Total available for sale, Carrying Value	261,559	274,409
Debt, Local Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	432,333	403,219
Total available for sale, Gross Unrealized Gains	32,684	18,350
Total available for sale, Gross Unrealized Losses	(647)	(1,339)
Total available for sale, Fair Value	464,370	420,230
Total available for sale, Carrying Value	464,370	420,230
Commercial Mortgage Backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	304,506	345,070
Total available for sale, Gross Unrealized Gains	5,214	1,304
Total available for sale, Gross Unrealized Losses	(346)	(3,708)
Total available for sale, Fair Value	309,374	342,666
Total available for sale, Carrying Value	309,374	342,666
Mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,602,651	1,609,446
Total available for sale, Gross Unrealized Gains	28,444	9,033
Total available for sale, Gross Unrealized Losses	(5,501)	(23,933)
Total available for sale, Fair Value	1,625,594	1,594,546
Total available for sale, Carrying Value	1,625,594	1,594,546
Asset-backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,802,588	2,462,303
Total available for sale, Gross Unrealized Gains	9,532	10,131
Total available for sale, Gross Unrealized Losses	(21,490)	(33,687)
Total available for sale, Fair Value	2,790,630	2,438,747
Total available for sale, Carrying Value	2,790,630	2,438,747
Industrial Property
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,260,073	2,295,778
Total available for sale, Gross Unrealized Gains	72,900	15,355
Total available for sale, Gross Unrealized Losses	(3,800)	(53,312)
Total available for sale, Fair Value	2,329,173	2,257,821
Total available for sale, Carrying Value	2,329,173	2,257,821
Debt Security, Corporate, US
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,447,589	1,502,427
Total available for sale, Gross Unrealized Gains	37,681	7,178
Total available for sale, Gross Unrealized Losses	(4,118)	(45,683)
Total available for sale, Fair Value	1,481,152	1,463,922
Total available for sale, Carrying Value	1,481,152	1,463,922
Public Utility, Bonds
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	325,762	330,326
Total available for sale, Gross Unrealized Gains	15,281	2,997
Total available for sale, Gross Unrealized Losses	(402)	(4,148)
Total available for sale, Fair Value	340,641	329,175
Total available for sale, Carrying Value	340,641	329,175
Other Security Investments
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	5,219	60,238
Total available for sale, Gross Unrealized Gains	230	322
Total available for sale, Gross Unrealized Losses	0	(167)
Total available for sale, Fair Value	5,449	60,393
Total available for sale, Carrying Value	5,449	60,393
Foreign government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	924,284	822,093
Total available for sale, Gross Unrealized Gains	16,465	11,753
Total available for sale, Gross Unrealized Losses	(93,673)	(25,111)
Total available for sale, Fair Value	847,076	808,735
Total available for sale, Carrying Value	$ 847,076	$ 808,735