Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands		Sep. 30, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		$ 88,177		$ 96,267
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(19,620)		18,453
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		107,797		114,720
Held-to-maturity Securities		88,177		96,267
Available-for-sale Debt Securities, Amortized Cost Basis		13,019,875		12,012,041
Available For Sale Securities Debt Securities Gross Unrealized Gain		542,348		427,492
Available For Sale Securities Debt Securities Gross Unrealized Losses		(33,729)		(91,406)
Available For Sale Securities Debt Securities Fair Value		13,528,494		12,348,127
Available-for-sale Securities, Debt Securities		13,528,494		12,348,127
Marketable Securities Fixed Maturities Gross Unrealized Gains		561,968		445,945
Fixed maturity securities		13,616,671		12,444,394
Marketable Securities Fixed Maturities Gross Unrealized Losses		33,729		91,406
Amortized Cost, Due in one year or less		1,032,042
Amortized Cost, Due after one year through five years		5,068,197
Amortized Cost, Due after five years through ten years		3,287,833
Amortized Cost, Due after ten years		2,462,938
Amortized Cost, Mortgaged-backed securities		1,257,042
Total investments in fixed maturity securities, Amortized Value		13,108,052		12,108,308
Fair Value, Due in one year or less		1,054,113
Fair Value, Due after one year through five years		5,246,081
Fair Value, Due after five years through ten years		3,500,463
Fair Value, Due after ten years		2,553,828
Fair Value, Mortgage-backed securities		1,281,806
Total investments in fixed maturity securities, Fair Value		13,636,291		12,462,847
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		909,765		799,839
Available For Sale Securities Debt Securities Gross Unrealized Gain		58,477		50,310
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,490)		(12,689)
Available For Sale Securities Debt Securities Fair Value		965,752		837,460
Available-for-sale Securities, Debt Securities		965,752		837,460
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		108,928		64,975
Available For Sale Securities Debt Securities Gross Unrealized Gain		1,032		875
Available For Sale Securities Debt Securities Gross Unrealized Losses		(916)		(128)
Available For Sale Securities Debt Securities Fair Value		109,044		65,722
Available-for-sale Securities, Debt Securities		109,044		65,722
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	[1]	1,131,545		1,126,382
Available For Sale Securities Debt Securities Gross Unrealized Gain	[1]	27,275		18,935
Available For Sale Securities Debt Securities Gross Unrealized Losses	[1]	(4,859)		(11,180)
Available For Sale Securities Debt Securities Fair Value		1,153,961	[1]	1,134,137
Available-for-sale Securities, Debt Securities	[1]	1,153,961		1,134,137
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		4,326,936		4,265,106
Available For Sale Securities Debt Securities Gross Unrealized Gain		237,364		201,342
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,585)		(6,269)
Available For Sale Securities Debt Securities Fair Value		4,561,715		4,460,179
Available-for-sale Securities, Debt Securities		4,561,715		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		530,541		645,092
Available For Sale Securities Debt Securities Gross Unrealized Gain		25,383		27,660
Available For Sale Securities Debt Securities Gross Unrealized Losses		(931)		(2,333)
Available For Sale Securities Debt Securities Fair Value		554,993		670,419
Available-for-sale Securities, Debt Securities		554,993		670,419
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,707,492		2,510,816
Available For Sale Securities Debt Securities Gross Unrealized Gain		124,159		102,909
Available For Sale Securities Debt Securities Gross Unrealized Losses		(1,767)		(3,737)
Available For Sale Securities Debt Securities Fair Value		2,829,884		2,609,988
Available-for-sale Securities, Debt Securities		2,829,884		2,609,988
Debt, State Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		528,380		583,456
Available For Sale Securities Debt Securities Gross Unrealized Gain		33,688		28,068
Available For Sale Securities Debt Securities Gross Unrealized Losses		(560)		(2,070)
Available For Sale Securities Debt Securities Fair Value		561,508		609,454
Available-for-sale Securities, Debt Securities		561,508		609,454
Revenue Subject to Refund [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		366,360		439,772
Available For Sale Securities Debt Securities Gross Unrealized Gain		31,900		32,056
Available For Sale Securities Debt Securities Gross Unrealized Losses		(9)		(31)
Available For Sale Securities Debt Securities Fair Value		398,251		471,797
Available-for-sale Securities, Debt Securities		398,251		471,797
Corporation [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		374,355		388,904
Available For Sale Securities Debt Securities Gross Unrealized Gain		15,424		14,039
Available For Sale Securities Debt Securities Gross Unrealized Losses		(235)		(402)
Available For Sale Securities Debt Securities Fair Value		389,544		402,541
Available-for-sale Securities, Debt Securities		389,544		402,541
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		3,746,954		3,403,953
Available For Sale Securities Debt Securities Gross Unrealized Gain		177,088		115,478
Available For Sale Securities Debt Securities Gross Unrealized Losses		(9,874)		(44,393)
Available For Sale Securities Debt Securities Fair Value		3,914,168		3,475,038
Available-for-sale Securities, Debt Securities		3,914,168		3,475,038
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,240,473		1,191,357
Available For Sale Securities Debt Securities Gross Unrealized Gain		28,307		19,810
Available For Sale Securities Debt Securities Gross Unrealized Losses		(5,775)		(11,308)
Available For Sale Securities Debt Securities Fair Value		1,263,005		1,199,859
Available-for-sale Securities, Debt Securities		1,263,005		1,199,859
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,265,206		1,706,694
Available For Sale Securities Debt Securities Gross Unrealized Gain		15,729		12,892
Available For Sale Securities Debt Securities Gross Unrealized Losses		(12,074)		(14,414)
Available For Sale Securities Debt Securities Fair Value		2,268,861		1,705,172
Available-for-sale Securities, Debt Securities		2,268,861		1,705,172
Industrial Property [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,168,312		1,976,393
Available For Sale Securities Debt Securities Gross Unrealized Gain		111,260		75,168
Available For Sale Securities Debt Securities Gross Unrealized Losses		(3,692)		(30,027)
Available For Sale Securities Debt Securities Fair Value		2,275,880		2,021,534
Available-for-sale Securities, Debt Securities		2,275,880		2,021,534
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,288,472		1,153,096
Available For Sale Securities Debt Securities Gross Unrealized Gain		52,419		31,744
Available For Sale Securities Debt Securities Gross Unrealized Losses		(5,384)		(11,819)
Available For Sale Securities Debt Securities Fair Value		1,335,507		1,173,021
Available-for-sale Securities, Debt Securities		1,335,507		1,173,021
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		199,941		192,857
Available For Sale Securities Debt Securities Gross Unrealized Gain		11,593		8,321
Available For Sale Securities Debt Securities Gross Unrealized Losses		(785)		(2,527)
Available For Sale Securities Debt Securities Fair Value		210,749		198,651
Available-for-sale Securities, Debt Securities		210,749		198,651
Other Security Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		90,229		81,607
Available For Sale Securities Debt Securities Gross Unrealized Gain		1,816		245
Available For Sale Securities Debt Securities Gross Unrealized Losses		(13)		(20)
Available For Sale Securities Debt Securities Fair Value		92,032		81,832
Available-for-sale Securities, Debt Securities		92,032		81,832
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		71,608		77,129
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(17,388)		16,246
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		88,996		93,375
Held-to-maturity Securities		71,608		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		16,569		19,138
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(2,232)		2,207
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		18,801		21,345
Held-to-maturity Securities		$ 16,569		$ 19,138

[1] Gross unrealized losses for residential mortgage-backed securities include $857,252 and $1,269,491 as of September 30, 2016 and December 31, 2015, respectively, related to the non-credit portion of other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income.