Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$ 14,224,376	$ 14,409,282
Available-for-sale investments, gross unrealized holding gains	3,452,316	2,664,201
Available-for-sale investments, gross unrealized holding losses	(51,838)	(97,050)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,931)
Available-for-sale investments, estimated fair value	17,624,854	16,973,502
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	129,631	259,379
Available-for-sale investments, gross unrealized holding gains	64	99
Available-for-sale investments, gross unrealized holding losses	(859)	(894)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	128,836	258,584
U.S. Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	359,492	418,457
Available-for-sale investments, gross unrealized holding gains	11,389	9,083
Available-for-sale investments, gross unrealized holding losses	(1,240)	(4,328)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	369,641	423,212
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	4,366,775	4,324,332
Available-for-sale investments, gross unrealized holding gains	199,013	145,678
Available-for-sale investments, gross unrealized holding losses	(12,789)	(41,805)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,552,999	4,428,205
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,395,157	1,306,324
Available-for-sale investments, gross unrealized holding gains	153,766	159,291
Available-for-sale investments, gross unrealized holding losses	(3,601)	(2,153)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,545,322	1,463,462
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,195,384	1,055,947
Available-for-sale investments, gross unrealized holding gains	8,353	3,953
Available-for-sale investments, gross unrealized holding losses	(12,327)	(19,544)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,191,410	1,040,356
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	799,872	779,503
Available-for-sale investments, gross unrealized holding gains	19,269	18,749
Available-for-sale investments, gross unrealized holding losses	(3,079)	(5,048)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,258)
Available-for-sale investments, estimated fair value	816,062	790,946
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	20,221	27,494
Available-for-sale investments, gross unrealized holding gains	7	2
Available-for-sale investments, gross unrealized holding losses	(72)	(158)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	20,156	27,338
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,248,550	1,420,298
Available-for-sale investments, gross unrealized holding gains	49,349	49,146
Available-for-sale investments, gross unrealized holding losses	(2,979)	(9,364)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(673)
Available-for-sale investments, estimated fair value	1,294,920	1,459,407
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	9,515,082	9,591,734
Available-for-sale investments, gross unrealized holding gains	441,210	386,001
Available-for-sale investments, gross unrealized holding losses	(36,946)	(83,294)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	(2,931)
Available-for-sale investments, estimated fair value	9,919,346	9,891,510
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	910,682	846,343
Available-for-sale investments, gross unrealized holding gains	1,103,007	857,063
Available-for-sale investments, gross unrealized holding losses	(3,418)	(5,596)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,010,271	1,697,810
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,803,123	1,635,105
Available-for-sale investments, gross unrealized holding gains	1,908,012	1,421,080
Available-for-sale investments, gross unrealized holding losses	(11,460)	(8,154)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	3,699,675	3,048,031
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,713,805	2,481,448
Available-for-sale investments, gross unrealized holding gains	3,011,019	2,278,143
Available-for-sale investments, gross unrealized holding losses	(14,878)	(13,750)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	5,709,946	4,745,841
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,995,489	2,336,100
Available-for-sale investments, gross unrealized holding gains	87	57
Available-for-sale investments, gross unrealized holding losses	(14)	(6)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$ 1,995,562	$ 2,336,151