Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 79,332	$ 88,526
Total held to maturity, Gross Unrealized Gains	15,726	14,146
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	95,058	102,672
Total held to maturity, Carrying Value	79,332	88,526
Total available for sale, Amortized Cost	13,242,580	12,869,809
Total available for sale, Gross Unrealized Gains	289,567	326,916
Total available for sale, Gross Unrealized Losses	(60,229)	(94,583)
Total available for sale, Fair Value	13,471,918	13,102,142
Total available for sale, Carrying Value	13,471,918	13,102,142
Total investment in fixed maturity securities, Amortized Cost	13,321,912	12,958,335
Total investments in fixed maturity securities, Gross Unrealized Gains	305,293	341,062
Total investments in fixed maturity securities, Gross Unrealized Losses	(60,229)	(94,583)
Total investments in fixed maturity securities, Fair Value	13,566,976	13,204,814
Total investments in fixed maturity securities, carrying value	13,551,250	13,190,668
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	819,345	858,773
Total available for sale, Gross Unrealized Gains	32,018	46,794
Total available for sale, Gross Unrealized Losses	(2,866)	(2,762)
Total available for sale, Fair Value	848,497	902,805
Total available for sale, Carrying Value	848,497	902,805
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	261,652	155,540
Total available for sale, Gross Unrealized Gains	1,521	304
Total available for sale, Gross Unrealized Losses	(2,628)	(2,981)
Total available for sale, Fair Value	260,545	152,863
Total available for sale, Carrying Value	260,545	152,863
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	372,748	496,187
Total available for sale, Gross Unrealized Gains	8,824	20,208
Total available for sale, Gross Unrealized Losses	(3,832)	(2,593)
Total available for sale, Fair Value	377,740	513,802
Total available for sale, Carrying Value	377,740	513,802
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	4,332,067	4,443,383
Total available for sale, Gross Unrealized Gains	125,601	116,558
Total available for sale, Gross Unrealized Losses	(12,574)	(40,438)
Total available for sale, Fair Value	4,445,094	4,519,503
Total available for sale, Carrying Value	4,445,094	4,519,503
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,043,629	1,034,301
Total available for sale, Gross Unrealized Gains	9,304	15,431
Total available for sale, Gross Unrealized Losses	(13,547)	(12,950)
Total available for sale, Fair Value	1,039,386	1,036,782
Total available for sale, Carrying Value	1,039,386	1,036,782
Total corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	4,302,007	3,967,795
Total available for sale, Gross Unrealized Gains	101,275	121,650
Total available for sale, Gross Unrealized Losses	(14,170)	(20,918)
Total available for sale, Fair Value	4,389,112	4,068,527
Total available for sale, Carrying Value	4,389,112	4,068,527
Debt Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,663,245	2,791,211
Total available for sale, Gross Unrealized Gains	53,512	58,559
Total available for sale, Gross Unrealized Losses	(10,027)	(26,315)
Total available for sale, Fair Value	2,706,730	2,823,455
Total available for sale, Carrying Value	2,706,730	2,823,455
Debt, State Government [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	439,358	524,682
Total available for sale, Gross Unrealized Gains	16,087	16,964
Total available for sale, Gross Unrealized Losses	(711)	(5,139)
Total available for sale, Fair Value	454,734	536,507
Total available for sale, Carrying Value	454,734	536,507
Revenue Subject to Refund [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	436,241	356,535
Total available for sale, Gross Unrealized Gains	22,701	19,181
Total available for sale, Gross Unrealized Losses	(9)	(165)
Total available for sale, Fair Value	458,933	375,551
Total available for sale, Carrying Value	458,933	375,551
Corporation [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	375,268	410,933
Total available for sale, Gross Unrealized Gains	10,059	6,172
Total available for sale, Gross Unrealized Losses	(860)	(6,452)
Total available for sale, Fair Value	384,467	410,653
Total available for sale, Carrying Value	384,467	410,653
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,305,281	1,189,841
Total available for sale, Gross Unrealized Gains	10,825	15,735
Total available for sale, Gross Unrealized Losses	(16,175)	(15,931)
Total available for sale, Fair Value	1,299,931	1,189,645
Total available for sale, Carrying Value	1,299,931	1,189,645
Asset-backed Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,111,132	1,913,830
Total available for sale, Gross Unrealized Gains	11,024	5,971
Total available for sale, Gross Unrealized Losses	(10,612)	(11,941)
Total available for sale, Fair Value	2,111,544	1,907,860
Total available for sale, Carrying Value	2,111,544	1,907,860
Industrial Property [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	2,574,400	2,315,567
Total available for sale, Gross Unrealized Gains	52,210	71,007
Total available for sale, Gross Unrealized Losses	(7,718)	(7,174)
Total available for sale, Fair Value	2,618,892	2,379,400
Total available for sale, Carrying Value	2,618,892	2,379,400
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	1,402,161	1,369,001
Total available for sale, Gross Unrealized Gains	37,744	39,543
Total available for sale, Gross Unrealized Losses	(5,138)	(11,270)
Total available for sale, Fair Value	1,434,767	1,397,274
Total available for sale, Carrying Value	1,434,767	1,397,274
Public Utility, Bonds [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	284,886	229,154
Total available for sale, Gross Unrealized Gains	11,316	10,801
Total available for sale, Gross Unrealized Losses	(1,248)	(2,411)
Total available for sale, Fair Value	294,954	237,544
Total available for sale, Carrying Value	294,954	237,544
Other Security Investments [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	40,560	54,073
Total available for sale, Gross Unrealized Gains	5	299
Total available for sale, Gross Unrealized Losses	(66)	(63)
Total available for sale, Fair Value	40,499	54,309
Total available for sale, Carrying Value	40,499	54,309
Debt, Local Government [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total available for sale, Amortized Cost	417,955	360,022
Total available for sale, Gross Unrealized Gains	23,242	15,682
Total available for sale, Gross Unrealized Losses	(967)	(2,367)
Total available for sale, Fair Value	440,230	373,337
Total available for sale, Carrying Value	440,230	373,337
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	13,450	15,944
Total held to maturity, Gross Unrealized Gains	1,227	1,693
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	14,677	17,637
Total held to maturity, Carrying Value	13,450	15,944
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	65,882	72,582
Total held to maturity, Gross Unrealized Gains	14,499	12,453
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	80,381	85,035
Total held to maturity, Carrying Value	$ 65,882	$ 72,582