Basis of Presentation - Balance Sheet Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Net
Commodity Sales	$ 475		$ 455
Joint Interest Billings	147		207
Other	90		103
Allowance for Doubtful Accounts	(14)		(17)
Total	698		748
Other Current Assets
Inventories, Materials and Supplies	52		66
Inventories, Crude Oil	34		16
Assets Held for Sale	0		629
Restricted Cash	1	$ 0	38	$ 30
Investment in Tamar Petroleum Ltd. Shares	165		0
Prepaid Expenses and Other Current Assets	57		31
Total	309		780
Other Noncurrent Assets
Equity Method Investments	295		305
Customer-Related Intangible Assets	318		0
Mutual Fund Investments	58		57
Net Deferred Income Tax Asset	25		25
Other Assets, Noncurrent	78		74
Total	774		461
Other Current Liabilities
Production and Ad Valorem Taxes	112		84
Commodity Derivative Liabilities	294		58
Income Taxes Payable	57		18
Asset Retirement Obligations	92		51
Interest Payable	87		67
Current Portion of Capital Lease Obligations	44		61
Liabilities Associated with Assets Held for Sale	0		55
Compensation and Benefits Payable	76		98
Other Liabilities, Current	123		86
Total	885		578
Other Noncurrent Liabilities
Deferred Compensation Liabilities	182		197
Asset Retirement Obligations	582		824
Production and Ad Valorem Taxes	60		69
Commodity Derivative Liabilities	100		15
Other Liabilities, Noncurrent	82		64
Total	1,075		$ 1,245
Liabilities Settled	309	53
Revisions of estimates	67	(56)
Accretion Expense	25	35
Liabilities Incurred	16	$ 83
Tamar and Dalit Fields
Other Noncurrent Liabilities
Ownership interest, percent			7.50%
Saddle Butte
Other Noncurrent Assets
Customer-Related Intangible Assets	$ 340