Fixed Assets and Asset Retirement Obligations - (Schedule of Reconciliation of Liability for Asset Retirement Obligations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement obligations, beginning balance	$ 213,726	$ 188,662
AROs arising from the acquisition and consolidation of a previously held equity method investment		20,940
Accretion expense	11,008	10,800
Revisions in timing and estimated costs of AROs	7,146	(2,254)
Acquisitions	131
Divestitures	(7,649)
Settlements	(26,415)	(4,422)
Other	240
Asset retirement obligations, ending balance	$ 198,187	$ 213,726