Asset Retirement Obligations (Rollforward) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
Beginning asset retirement obligations		$ 145,696	$ 128,095
Liabilities incurred and assumed during period		5,383	9,628
Revisions in estimated retirement obligations		6,238	5,238
Liabilities settled and sold during period		(19,878)	(6,914)
Accretion expense		11,681	9,649
Ending asset retirement obligations		149,120	145,696
Less: current asset retirement obligations	[1]	(2,313)	(6,777)
Long-term asset retirement obligations		$ 146,807	$ 138,919

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