Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Variable Interest Entity [Line Items]
Noncontrolling interests on condensed consolidated balance sheets	$ 253		$ 253		$ 257
Net income attributable to noncontrolling interests, Trust	1	$ 1	3	$ 1
VIE, cash and cash equivalents	5		5		882
VIE. proved oil and natural gas properties	68,095		68,095		66,451
VIE. accumulated depreciation, depletion and amortization	63,375		63,375		62,726
VIE, other current liabilities	1,397		1,397
Variable Interest Entities, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
VIE, cash and cash equivalents	1		1		1
VIE. proved oil and natural gas properties	488		488		488
VIE. accumulated depreciation, depletion and amortization	460		460		$ 458
VIE, other current liabilities	$ 4		$ 4