Discontinued Operations - Reported in Consolidated Statements of Income and Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Income
Impairments		$ 75	$ 229	$ 67
Cash Flow from Discontinued Operations
Net increase in cash and cash equivalents of discontinued operations		0	130	238
Canadian Business [Member]
Consolidated Statements of Income
Impairments	$ 6,600
Canadian Business [Member] | Discontinued Operations, Disposed of by Sale [Member]
Consolidated Statements of Income
Total revenue and other income		0	431	863
Net gain (loss) on disposal of assets		0	(43)	0
Total revenues and other income		0	388	863
Production		0	254	601
Exploration		0	0	7
Depreciation, depletion and amortization		0	40	239
Impairments		0	6,636	0
Other		0	25	87
Total costs and expenses		0	6,955	934
Pretax income (loss) from discontinued operations		0	(6,567)	(71)
Provision (benefit) for income taxes		0	(1,674)	(18)
Income (loss) from discontinued operations		0	(4,893)	(53)
Cash Flow from Discontinued Operations
Operating activities		0	141	177
Investing activities		0	(13)	(41)
Changes in cash included in current assets held for sale		0	2	100
Net increase in cash and cash equivalents of discontinued operations		$ 0	$ 130	$ 236