Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements) (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 6,022,315	$ 5,791,724				$ 5,791,724		$ 5,799,173
Revenues	659,436	675,817	$ 657,225	$ 676,087	$ 582,717	2,591,846	$ 2,481,890
Costs and Expenses	(615,222)				$ (572,464)
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				$ 5,791,724
Revenues	1,300
Costs and Expenses	(3,000)
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