Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 6,061,932		$ 6,061,932			$ 5,799,173	$ 5,791,724
Revenues	(672,677)	$ (676,087)	(1,332,113)	$ (1,258,804)
Costs and Expenses	(607,051)	$ (626,346)	(1,222,273)	$ (1,198,810)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ 7,449
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings							$ 5,791,724
Revenues			(1,300)
Costs and Expenses	$ (3,800)		$ (800)
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 194,900