Asset Retirement Obligations (Rollforward) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation Roll Forward [Roll Forward]
Beginning asset retirement obligations		$ 166,310	$ 149,120
Liabilities incurred and assumed during period		2,201	2,698
Revisions in estimated retirement obligations		2,298	6,867
Liabilities settled and sold during period		(9,481)	(5,617)
Accretion expense		15,257	13,242
Ending asset retirement obligations		176,585	166,310
Less: current asset retirement obligations	[1]	(2,115)	(554)
Long-term asset retirement obligations		$ 174,470	$ 165,756

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