Asset Retirement Obligations (Rollforward) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation Roll Forward [Roll Forward]
Beginning asset retirement obligations		$ 149,120	$ 145,696
Liabilities incurred and assumed during period		2,698	5,383
Revisions in estimated retirement obligations		6,867	6,238
Liabilities settled and sold during period		(5,617)	(19,878)
Accretion expense		13,242	11,681
Ending asset retirement obligations		166,310	149,120
Less: current asset retirement obligations	[1]	(554)	(2,313)
Long-term asset retirement obligations		$ 165,756	$ 146,807

[1] Included in “Accounts payable and accrued liabilities” in our Consolidated Balance Sheets.