Asset Retirement Obligations (Schedule of Reconciliation of Changes in the Asset Retirement Obligations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 272,538	$ 288,977
Accretion expense	18,922	20,111
Payments	(10,467)	(11,637)
Revisions in estimated cash flows	(20,529)	(25,427)
Other	(1,641)	514
Balance at end of period	$ 258,823	$ 272,538