Asset Retirement Obligations (Rollforward) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation Roll Forward [Roll Forward]
Beginning asset retirement obligations		$ 176,585	$ 166,310
Liabilities incurred and assumed during period		4,354	2,201
Revisions in estimated retirement obligations		9,206	2,298
Liabilities settled and sold during period		(24,342)	(9,481)
Accretion expense		15,957	15,257
Ending asset retirement obligations		181,760	176,585
Less: current asset retirement obligations	[1]	(4,652)	(2,115)
Long-term asset retirement obligations		$ 177,108	$ 174,470

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