Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands		Mar. 31, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		$ 88,947		$ 88,526
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		15,025		14,146
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		103,972		102,672
Held-to-maturity Securities		88,947		88,526
Available-for-sale Debt Securities, Amortized Cost Basis		13,061,342		12,869,809
Available For Sale Securities Debt Securities Gross Unrealized Gain		341,982		326,916
Available For Sale Securities Debt Securities Gross Unrealized Losses		(69,292)		(94,583)
Available For Sale Securities Debt Securities Fair Value		13,334,032		13,102,142
Available-for-sale Securities, Debt Securities		13,334,032		13,102,142
Marketable Securities Fixed Maturities Gross Unrealized Gains		357,007		341,062
Fixed maturity securities		13,422,979		13,190,668
Marketable Securities Fixed Maturities Gross Unrealized Losses		69,292		94,583
Amortized Cost, Due in one year or less		1,033,358
Amortized Cost, Due after one year through five years		5,160,693
Amortized Cost, Due after five years through ten years		3,273,799
Amortized Cost, Due after ten years		2,454,761
Amortized Cost, Mortgaged-backed securities		1,227,678
Total investments in fixed maturity securities, Amortized Value		13,150,289		12,958,335
Fair Value, Due in one year or less		1,049,761
Fair Value, Due after one year through five years		5,288,826
Fair Value, Due after five years through ten years		3,386,314
Fair Value, Due after ten years		2,484,502
Fair Value, Mortgage-backed securities		1,228,601
Total investments in fixed maturity securities, Fair Value		13,438,004		13,204,814
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		864,556		858,773
Available For Sale Securities Debt Securities Gross Unrealized Gain		38,692		46,794
Available For Sale Securities Debt Securities Gross Unrealized Losses		(1,577)		(2,762)
Available For Sale Securities Debt Securities Fair Value		901,671		902,805
Available-for-sale Securities, Debt Securities		901,671		902,805
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		144,583		155,540
Available For Sale Securities Debt Securities Gross Unrealized Gain		347		304
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,126)		(2,981)
Available For Sale Securities Debt Securities Fair Value		142,804		152,863
Available-for-sale Securities, Debt Securities		142,804		152,863
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	[1]	1,067,779		1,034,301
Available For Sale Securities Debt Securities Gross Unrealized Gain	[1]	14,081		15,431
Available For Sale Securities Debt Securities Gross Unrealized Losses	[1]	(13,030)		(12,950)
Available For Sale Securities Debt Securities Fair Value		1,068,830	[1]	1,036,782
Available-for-sale Securities, Debt Securities	[1]	1,068,830		1,036,782
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		4,350,677		4,443,383
Available For Sale Securities Debt Securities Gross Unrealized Gain		136,978		116,558
Available For Sale Securities Debt Securities Gross Unrealized Losses		(26,569)		(40,438)
Available For Sale Securities Debt Securities Fair Value		4,461,086		4,519,503
Available-for-sale Securities, Debt Securities		4,461,086		4,519,503
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		472,112		496,187
Available For Sale Securities Debt Securities Gross Unrealized Gain		16,784		20,208
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,220)		(2,593)
Available For Sale Securities Debt Securities Fair Value		486,676		513,802
Available-for-sale Securities, Debt Securities		486,676		513,802
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,759,063		2,791,211
Available For Sale Securities Debt Securities Gross Unrealized Gain		69,226		58,559
Available For Sale Securities Debt Securities Gross Unrealized Losses		(15,887)		(26,315)
Available For Sale Securities Debt Securities Fair Value		2,812,402		2,823,455
Available-for-sale Securities, Debt Securities		2,812,402		2,823,455
Debt, State Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		504,312		524,682
Available For Sale Securities Debt Securities Gross Unrealized Gain		18,952		16,964
Available For Sale Securities Debt Securities Gross Unrealized Losses		(3,295)		(5,139)
Available For Sale Securities Debt Securities Fair Value		519,969		536,507
Available-for-sale Securities, Debt Securities		519,969		536,507
Revenue Subject to Refund [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		349,697		356,535
Available For Sale Securities Debt Securities Gross Unrealized Gain		20,013		19,181
Available For Sale Securities Debt Securities Gross Unrealized Losses		(146)		(165)
Available For Sale Securities Debt Securities Fair Value		369,564		375,551
Available-for-sale Securities, Debt Securities		369,564		375,551
Corporation [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		376,664		410,933
Available For Sale Securities Debt Securities Gross Unrealized Gain		8,290		6,172
Available For Sale Securities Debt Securities Gross Unrealized Losses		(5,639)		(6,452)
Available For Sale Securities Debt Securities Fair Value		379,315		410,653
Available-for-sale Securities, Debt Securities		379,315		410,653
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		4,131,823		3,967,795
Available For Sale Securities Debt Securities Gross Unrealized Gain		127,263		121,650
Available For Sale Securities Debt Securities Gross Unrealized Losses		(14,290)		(20,918)
Available For Sale Securities Debt Securities Fair Value		4,244,796		4,068,527
Available-for-sale Securities, Debt Securities		4,244,796		4,068,527
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,212,362		1,189,841
Available For Sale Securities Debt Securities Gross Unrealized Gain		14,428		15,735
Available For Sale Securities Debt Securities Gross Unrealized Losses		(15,156)		(15,931)
Available For Sale Securities Debt Securities Fair Value		1,211,634		1,189,645
Available-for-sale Securities, Debt Securities		1,211,634		1,189,645
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,029,812		1,913,830
Available For Sale Securities Debt Securities Gross Unrealized Gain		7,837		5,971
Available For Sale Securities Debt Securities Gross Unrealized Losses		(9,480)		(11,941)
Available For Sale Securities Debt Securities Fair Value		2,028,169		1,907,860
Available-for-sale Securities, Debt Securities		2,028,169		1,907,860
Industrial Property [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,391,507		2,315,567
Available For Sale Securities Debt Securities Gross Unrealized Gain		70,783		71,007
Available For Sale Securities Debt Securities Gross Unrealized Losses		(5,330)		(7,174)
Available For Sale Securities Debt Securities Fair Value		2,456,960		2,379,400
Available-for-sale Securities, Debt Securities		2,456,960		2,379,400
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,445,707		1,369,001
Available For Sale Securities Debt Securities Gross Unrealized Gain		45,431		39,543
Available For Sale Securities Debt Securities Gross Unrealized Losses		(6,881)		(11,270)
Available For Sale Securities Debt Securities Fair Value		1,484,257		1,397,274
Available-for-sale Securities, Debt Securities		1,484,257		1,397,274
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		238,801		229,154
Available For Sale Securities Debt Securities Gross Unrealized Gain		10,578		10,801
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,049)		(2,411)
Available For Sale Securities Debt Securities Fair Value		247,330		237,544
Available-for-sale Securities, Debt Securities		247,330		237,544
Other Security Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		55,808		54,073
Available For Sale Securities Debt Securities Gross Unrealized Gain		471		299
Available For Sale Securities Debt Securities Gross Unrealized Losses		(30)		(63)
Available For Sale Securities Debt Securities Fair Value		56,249		54,309
Available-for-sale Securities, Debt Securities		56,249		54,309
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		73,631		72,582
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		13,374		12,453
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		87,005		85,035
Held-to-maturity Securities		73,631		72,582
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		15,316		15,944
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		1,651		1,693
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		16,967		17,637
Held-to-maturity Securities		$ 15,316		$ 15,944

[1] Gross unrealized losses for residential mortgage-backed securities include $717,960 and $818,691 as of March 31, 2017 and December 31, 2016, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income.