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, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		$ 96,669		$ 96,267
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(19,810)		(18,453)
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		116,479		114,720
Held-to-maturity Securities		96,669		96,267
Available-for-sale Debt Securities, Amortized Cost Basis		12,218,217		12,012,041
Available For Sale Securities Debt Securities Gross Unrealized Gain		507,661		427,492
Available For Sale Securities Debt Securities Gross Unrealized Losses		(77,111)		(91,406)
Available For Sale Securities Debt Securities Fair Value		12,648,767		12,348,127
Available-for-sale Securities, Debt Securities		12,648,767		12,348,127
Marketable Securities Fixed Maturities Gross Unrealized Gains		527,471		445,945
Fixed maturity securities		12,745,436		12,444,394
Marketable Securities Fixed Maturities Gross Unrealized Losses		77,111		91,406
Amortized Cost, Due in one year or less		812,491
Amortized Cost, Due after one year through five years		4,360,294
Amortized Cost, Due after five years through ten years		3,482,275
Amortized Cost, Due after ten years		2,508,406
Amortized Cost, Mortgaged-backed securities		1,151,420
Total investments in fixed maturity securities, Amortized Value		12,314,886		12,108,308
Fair Value, Due in one year or less		822,851
Fair Value, Due after one year through five years		4,522,431
Fair Value, Due after five years through ten years		3,659,124
Fair Value, Due after ten years		2,584,816
Fair Value, Mortgage-backed securities		1,176,024
Total investments in fixed maturity securities, Fair Value		12,765,246		12,462,847
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		849,800		799,839
Available For Sale Securities Debt Securities Gross Unrealized Gain		54,580		50,310
Available For Sale Securities Debt Securities Gross Unrealized Losses		(6,461)		(12,689)
Available For Sale Securities Debt Securities Fair Value		897,919		837,460
Available-for-sale Securities, Debt Securities		897,919		837,460
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		65,138		64,975
Available For Sale Securities Debt Securities Gross Unrealized Gain		778		875
Available For Sale Securities Debt Securities Gross Unrealized Losses		(616)		(128)
Available For Sale Securities Debt Securities Fair Value		65,300		65,722
Available-for-sale Securities, Debt Securities		65,300		65,722
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	[1]	1,067,877		1,126,382
Available For Sale Securities Debt Securities Gross Unrealized Gain	[1]	27,520		18,935
Available For Sale Securities Debt Securities Gross Unrealized Losses	[1]	(5,548)		(11,180)
Available For Sale Securities Debt Securities Fair Value		1,089,849	[1]	1,134,137
Available-for-sale Securities, Debt Securities	[1]	1,089,849		1,134,137
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		4,361,448		4,265,106
Available For Sale Securities Debt Securities Gross Unrealized Gain		227,819		201,342
Available For Sale Securities Debt Securities Gross Unrealized Losses		(4,499)		(6,269)
Available For Sale Securities Debt Securities Fair Value		4,584,768		4,460,179
Available-for-sale Securities, Debt Securities		4,584,768		4,460,179
U.S. government and government agency
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		571,330		645,092
Available For Sale Securities Debt Securities Gross Unrealized Gain		27,602		27,660
Available For Sale Securities Debt Securities Gross Unrealized Losses		(981)		(2,333)
Available For Sale Securities Debt Securities Fair Value		597,951		670,419
Available-for-sale Securities, Debt Securities		597,951		670,419
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,613,723		2,510,816
Available For Sale Securities Debt Securities Gross Unrealized Gain		120,174		102,909
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,047)		(3,737)
Available For Sale Securities Debt Securities Fair Value		2,731,850		2,609,988
Available-for-sale Securities, Debt Securities		2,731,850		2,609,988
Debt, State Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		595,465		583,456
Available For Sale Securities Debt Securities Gross Unrealized Gain		30,624		28,068
Available For Sale Securities Debt Securities Gross Unrealized Losses		(1,817)		(2,070)
Available For Sale Securities Debt Securities Fair Value		624,272		609,454
Available-for-sale Securities, Debt Securities		624,272		609,454
Revenue Subject to Refund [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		407,586		439,772
Available For Sale Securities Debt Securities Gross Unrealized Gain		32,630		32,056
Available For Sale Securities Debt Securities Gross Unrealized Losses		0		(31)
Available For Sale Securities Debt Securities Fair Value		440,216		471,797
Available-for-sale Securities, Debt Securities		440,216		471,797
Corporation [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		388,408		388,904
Available For Sale Securities Debt Securities Gross Unrealized Gain		15,034		14,039
Available For Sale Securities Debt Securities Gross Unrealized Losses		(618)		(402)
Available For Sale Securities Debt Securities Fair Value		402,824		402,541
Available-for-sale Securities, Debt Securities		402,824		402,541
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		3,447,323		3,403,953
Available For Sale Securities Debt Securities Gross Unrealized Gain		157,413		115,478
Available For Sale Securities Debt Securities Gross Unrealized Losses		(36,160)		(44,393)
Available For Sale Securities Debt Securities Fair Value		3,568,576		3,475,038
Available-for-sale Securities, Debt Securities		3,568,576		3,475,038
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,133,015		1,191,357
Available For Sale Securities Debt Securities Gross Unrealized Gain		28,298		19,810
Available For Sale Securities Debt Securities Gross Unrealized Losses		(6,164)		(11,308)
Available For Sale Securities Debt Securities Fair Value		1,155,149		1,199,859
Available-for-sale Securities, Debt Securities		1,155,149		1,199,859
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,855,301		1,706,694
Available For Sale Securities Debt Securities Gross Unrealized Gain		11,949		12,892
Available For Sale Securities Debt Securities Gross Unrealized Losses		(22,846)		(14,414)
Available For Sale Securities Debt Securities Fair Value		1,844,404		1,705,172
Available-for-sale Securities, Debt Securities		1,844,404		1,705,172
Industrial Property [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,007,460		1,976,393
Available For Sale Securities Debt Securities Gross Unrealized Gain		106,650		75,168
Available For Sale Securities Debt Securities Gross Unrealized Losses		(20,538)		(30,027)
Available For Sale Securities Debt Securities Fair Value		2,093,572		2,021,534
Available-for-sale Securities, Debt Securities		2,093,572		2,021,534
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,176,238		1,153,096
Available For Sale Securities Debt Securities Gross Unrealized Gain		39,566		31,744
Available For Sale Securities Debt Securities Gross Unrealized Losses		(13,664)		(11,819)
Available For Sale Securities Debt Securities Fair Value		1,202,140		1,173,021
Available-for-sale Securities, Debt Securities		1,202,140		1,173,021
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		197,336		192,857
Available For Sale Securities Debt Securities Gross Unrealized Gain		10,771		8,321
Available For Sale Securities Debt Securities Gross Unrealized Losses		(1,958)		(2,527)
Available For Sale Securities Debt Securities Fair Value		206,149		198,651
Available-for-sale Securities, Debt Securities		206,149		198,651
Other Security Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		66,289		81,607
Available For Sale Securities Debt Securities Gross Unrealized Gain		426		245
Available For Sale Securities Debt Securities Gross Unrealized Losses		0		(20)
Available For Sale Securities Debt Securities Fair Value		66,715		81,832
Available-for-sale Securities, Debt Securities		66,715		81,832
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		78,264		77,129
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(17,340)		16,246
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		95,604		93,375
Held-to-maturity Securities		78,264		77,129
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		18,405		19,138
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(2,470)		(2,207)
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		20,875		21,345
Held-to-maturity Securities		$ 18,405		$ 19,138

[1] Gross unrealized losses for residential mortgage-backed securities include $1,324,214 and $1,269,491 as of March 31, 2016 and December 31, 2015, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income.