Basis of Presentation - Balance Sheet Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Net
Commodity Sales	$ 460		$ 455
Joint Interest Billings	210		207
Other	89		103
Allowance for Doubtful Accounts	(16)		(17)
Total	743		748
Other Current Assets
Inventories, Materials and Supplies	46		66
Inventories, Crude Oil	27		16
Commodity Derivative Assets	29		2
Assets Held for Sale	40		629
Restricted Cash	0	$ 0	38	$ 30
Prepaid Expenses and Other Current Assets	45		29
Total	187		780
Other Noncurrent Assets
Equity Method Investments	357		305
Customer-Related Intangible Assets	326		0
Investment in Tamar Petroleum Ltd	150		0
Mutual Fund Investments	57		57
Net Deferred Income Tax Asset	25		25
Other Assets, Noncurrent	69		74
Total	984		461
Other Current Liabilities
Production and Ad Valorem Taxes	111		84
Commodity Derivative Liabilities	250		58
Income Taxes Payable	5		18
Asset Retirement Obligations	92		51
Interest Payable	64		67
Current Portion of Capital Lease Obligations	47		61
Liabilities Associated with Assets Held for Sale	0		55
Compensation and Benefits Payable	66		98
Other Liabilities, Current	110		86
Total	745		578
Other Noncurrent Liabilities
Deferred Compensation Liabilities	180		197
Asset Retirement Obligations	543		824
Marcellus Shale Firm Transportation Commitment	71		76
Production and Ad Valorem Taxes	39		69
Commodity Derivative Liabilities	85		15
Other Liabilities, Noncurrent	77		64
Total	995		$ 1,245
Tamar and Dalit Fields
Other Noncurrent Liabilities
Ownership interest, percent			7.50%
Other Current Liabilities
Other Noncurrent Liabilities
Firm transportation liability	12	$ 14
CNX Midstream Partners
Other Noncurrent Assets
Equity Method Investments	49
Saddle Butte
Other Noncurrent Assets
Customer-Related Intangible Assets	340
Other Noncurrent Liabilities
Accumulated amortization	$ 14