Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Interest paid (net of amounts capitalized)	$ (270)	$ (379)	$ (375)
Income taxes paid to taxing authorities	(323)	(391)	(84)
Commercial paper:
Asset Retirement Obligation, Period Increase (Decrease)	(183)	(202)	110
Noncash investing and financing activities:
Asset Retirement Obligation, Liabilities Transferred	(82)	14	40
Notes receivable	0	748
Receivable Related To Disposal Of Assets			0
Increase in capital expenditure accrual	$ 250	329	$ 155
Foreign Tax Authority [Member] | UK Government [Member]
Income Tax Examination [Line Items]
Income Tax Examination, Penalties and Interest Expense		$ 108