Supplemental Cash Flow Information (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Aug. 07, 2018	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Payments of interest and commitment fees	$ 158,200	$ 126,900
Interest paid, capitalized	2,700	13,800
Incurred liabilities for fixed and intangible asset additions	16,600	$ 25,700
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Current assets held for sale	255,519			$ 0
Non-Core Assets Powder River Basin [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Option payment on assets held for sale			$ 30,000
Current assets held for sale	$ 255,500