Consolidated Statements of Cash Flows Supplemental Disclosures (Non-cash Investing and Financing Activities) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Non-cash additions for purchases of property and equipment	$ 13,676	$ 11,372
Net asset retirement obligation additions (deletions) to property and equipment	$ 986	$ (2,279)