Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands		Jun. 30, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		$ 88,019		$ 96,267
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(21,758)		(18,453)
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		109,777		114,720
Held-to-maturity Securities		88,019		96,267
Available-for-sale Debt Securities, Amortized Cost Basis		12,311,389		12,012,041
Available For Sale Securities Debt Securities Gross Unrealized Gain		599,026		427,492
Available For Sale Securities Debt Securities Gross Unrealized Losses		(43,512)		(91,406)
Available For Sale Securities Debt Securities Fair Value		12,866,903		12,348,127
Available-for-sale Securities, Debt Securities		12,866,903		12,348,127
Marketable Securities Fixed Maturities Gross Unrealized Gains		620,784		445,945
Fixed maturity securities		12,954,922		12,444,394
Marketable Securities Fixed Maturities Gross Unrealized Losses		43,512		91,406
Amortized Cost, Due in one year or less		844,750
Amortized Cost, Due after one year through five years		4,599,884
Amortized Cost, Due after five years through ten years		3,321,186
Amortized Cost, Due after ten years		2,508,153
Amortized Cost, Mortgaged-backed securities		1,125,435
Total investments in fixed maturity securities, Amortized Value		12,399,408		12,108,308
Fair Value, Due in one year or less		878,691
Fair Value, Due after one year through five years		4,784,106
Fair Value, Due after five years through ten years		3,544,108
Fair Value, Due after ten years		2,614,167
Fair Value, Mortgage-backed securities		1,155,608
Total investments in fixed maturity securities, Fair Value		12,976,680		12,462,847
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		836,109		799,839
Available For Sale Securities Debt Securities Gross Unrealized Gain		66,413		50,310
Available For Sale Securities Debt Securities Gross Unrealized Losses		(4,183)		(12,689)
Available For Sale Securities Debt Securities Fair Value		898,339		837,460
Available-for-sale Securities, Debt Securities		898,339		837,460
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		97,524		64,975
Available For Sale Securities Debt Securities Gross Unrealized Gain		970		875
Available For Sale Securities Debt Securities Gross Unrealized Losses		(770)		(128)
Available For Sale Securities Debt Securities Fair Value		97,724		65,722
Available-for-sale Securities, Debt Securities		97,724		65,722
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	[1]	1,010,596		1,126,382
Available For Sale Securities Debt Securities Gross Unrealized Gain	[1]	31,157		18,935
Available For Sale Securities Debt Securities Gross Unrealized Losses	[1]	(3,630)		(11,180)
Available For Sale Securities Debt Securities Fair Value		1,038,123	[1]	1,134,137
Available-for-sale Securities, Debt Securities	[1]	1,038,123		1,134,137
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		4,352,454		4,265,106
Available For Sale Securities Debt Securities Gross Unrealized Gain		266,525		201,342
Available For Sale Securities Debt Securities Gross Unrealized Losses		(2,570)		(6,269)
Available For Sale Securities Debt Securities Fair Value		4,616,409		4,460,179
Available-for-sale Securities, Debt Securities		4,616,409		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		532,492		645,092
Available For Sale Securities Debt Securities Gross Unrealized Gain		28,680		27,660
Available For Sale Securities Debt Securities Gross Unrealized Losses		(682)		(2,333)
Available For Sale Securities Debt Securities Fair Value		560,490		670,419
Available-for-sale Securities, Debt Securities		560,490		670,419
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,595,942		2,510,816
Available For Sale Securities Debt Securities Gross Unrealized Gain		141,661		102,909
Available For Sale Securities Debt Securities Gross Unrealized Losses		(1,038)		(3,737)
Available For Sale Securities Debt Securities Fair Value		2,736,565		2,609,988
Available-for-sale Securities, Debt Securities		2,736,565		2,609,988
Debt, State Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		577,408		583,456
Available For Sale Securities Debt Securities Gross Unrealized Gain		35,119		28,068
Available For Sale Securities Debt Securities Gross Unrealized Losses		(987)		(2,070)
Available For Sale Securities Debt Securities Fair Value		611,540		609,454
Available-for-sale Securities, Debt Securities		611,540		609,454
Revenue Subject to Refund [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		448,185		439,772
Available For Sale Securities Debt Securities Gross Unrealized Gain		36,443		32,056
Available For Sale Securities Debt Securities Gross Unrealized Losses		0		(31)
Available For Sale Securities Debt Securities Fair Value		484,628		471,797
Available-for-sale Securities, Debt Securities		484,628		471,797
Corporation [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		387,006		388,904
Available For Sale Securities Debt Securities Gross Unrealized Gain		17,599		14,039
Available For Sale Securities Debt Securities Gross Unrealized Losses		(545)		(402)
Available For Sale Securities Debt Securities Fair Value		404,060		402,541
Available-for-sale Securities, Debt Securities		404,060		402,541
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		3,562,935		3,403,953
Available For Sale Securities Debt Securities Gross Unrealized Gain		191,948		115,478
Available For Sale Securities Debt Securities Gross Unrealized Losses		(15,341)		(44,393)
Available For Sale Securities Debt Securities Fair Value		3,739,542		3,475,038
Available-for-sale Securities, Debt Securities		3,739,542		3,475,038
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,108,120		1,191,357
Available For Sale Securities Debt Securities Gross Unrealized Gain		32,127		19,810
Available For Sale Securities Debt Securities Gross Unrealized Losses		(4,400)		(11,308)
Available For Sale Securities Debt Securities Fair Value		1,135,847		1,199,859
Available-for-sale Securities, Debt Securities		1,135,847		1,199,859
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,919,279		1,706,694
Available For Sale Securities Debt Securities Gross Unrealized Gain		13,333		12,892
Available For Sale Securities Debt Securities Gross Unrealized Losses		(16,336)		(14,414)
Available For Sale Securities Debt Securities Fair Value		1,916,276		1,705,172
Available-for-sale Securities, Debt Securities		1,916,276		1,705,172
Industrial Property [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		2,093,170		1,976,393
Available For Sale Securities Debt Securities Gross Unrealized Gain		128,048		75,168
Available For Sale Securities Debt Securities Gross Unrealized Losses		(6,795)		(30,027)
Available For Sale Securities Debt Securities Fair Value		2,214,423		2,021,534
Available-for-sale Securities, Debt Securities		2,214,423		2,021,534
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		1,196,178		1,153,096
Available For Sale Securities Debt Securities Gross Unrealized Gain		50,925		31,744
Available For Sale Securities Debt Securities Gross Unrealized Losses		(7,565)		(11,819)
Available For Sale Securities Debt Securities Fair Value		1,239,538		1,173,021
Available-for-sale Securities, Debt Securities		1,239,538		1,173,021
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		203,145		192,857
Available For Sale Securities Debt Securities Gross Unrealized Gain		12,391		8,321
Available For Sale Securities Debt Securities Gross Unrealized Losses		(972)		(2,527)
Available For Sale Securities Debt Securities Fair Value		214,564		198,651
Available-for-sale Securities, Debt Securities		214,564		198,651
Other Security Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis		70,442		81,607
Available For Sale Securities Debt Securities Gross Unrealized Gain		584		245
Available For Sale Securities Debt Securities Gross Unrealized Losses		(9)		(20)
Available For Sale Securities Debt Securities Fair Value		71,017		81,832
Available-for-sale Securities, Debt Securities		71,017		81,832
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		70,704		77,129
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(19,312)		16,246
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		90,016		93,375
Held-to-maturity Securities		70,704		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		17,315		19,138
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		(2,446)		(2,207)
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		0		0
Held-to-maturity Securities, Fair Value		19,761		21,345
Held-to-maturity Securities		$ 17,315		$ 19,138

[1] Gross unrealized losses for residential mortgage-backed securities include $985,213 and $1,269,491 as of June 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.