Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$ 14,027,025	$ 14,409,282
Available-for-sale investments, gross unrealized holding gains	3,126,385	2,664,201
Available-for-sale investments, gross unrealized holding losses	(54,675)	(97,050)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,931)
Available-for-sale investments, estimated fair value	17,098,735	16,973,502
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	138,404	259,379
Available-for-sale investments, gross unrealized holding gains	81	99
Available-for-sale investments, gross unrealized holding losses	(844)	(894)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	137,641	258,584
U.S. Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	372,560	418,457
Available-for-sale investments, gross unrealized holding gains	10,745	9,083
Available-for-sale investments, gross unrealized holding losses	(1,426)	(4,328)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	381,879	423,212
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	4,477,463	4,324,332
Available-for-sale investments, gross unrealized holding gains	196,535	145,678
Available-for-sale investments, gross unrealized holding losses	(20,632)	(41,805)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,653,366	4,428,205
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,310,376	1,306,324
Available-for-sale investments, gross unrealized holding gains	143,572	159,291
Available-for-sale investments, gross unrealized holding losses	(2,231)	(2,153)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,451,717	1,463,462
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,188,439	1,055,947
Available-for-sale investments, gross unrealized holding gains	8,014	3,953
Available-for-sale investments, gross unrealized holding losses	(13,054)	(19,544)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,183,399	1,040,356
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	830,679	779,503
Available-for-sale investments, gross unrealized holding gains	21,786	18,749
Available-for-sale investments, gross unrealized holding losses	(3,690)	(5,048)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,258)
Available-for-sale investments, estimated fair value	848,775	790,946
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	37,856	27,494
Available-for-sale investments, gross unrealized holding gains	25	2
Available-for-sale investments, gross unrealized holding losses	(84)	(158)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	37,797	27,338
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,313,879	1,420,298
Available-for-sale investments, gross unrealized holding gains	48,340	49,146
Available-for-sale investments, gross unrealized holding losses	(3,301)	(9,364)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(673)
Available-for-sale investments, estimated fair value	1,358,918	1,459,407
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	9,669,656	9,591,734
Available-for-sale investments, gross unrealized holding gains	429,098	386,001
Available-for-sale investments, gross unrealized holding losses	(45,262)	(83,294)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,931)
Available-for-sale investments, estimated fair value	10,053,492	9,891,510
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	888,805	846,343
Available-for-sale investments, gross unrealized holding gains	976,110	857,063
Available-for-sale investments, gross unrealized holding losses	(586)	(5,596)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,864,329	1,697,810
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,764,205	1,635,105
Available-for-sale investments, gross unrealized holding gains	1,721,110	1,421,080
Available-for-sale investments, gross unrealized holding losses	(8,817)	(8,154)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	3,476,498	3,048,031
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,653,010	2,481,448
Available-for-sale investments, gross unrealized holding gains	2,697,220	2,278,143
Available-for-sale investments, gross unrealized holding losses	(9,403)	(13,750)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	5,340,827	4,745,841
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,704,359	2,336,100
Available-for-sale investments, gross unrealized holding gains	67	57
Available-for-sale investments, gross unrealized holding losses	(10)	(6)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$ 1,704,416	$ 2,336,151