Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net cash provided by (used in) operating activities included:
Interest paid (net of amounts capitalized)	$ (52)	$ (64)
Income taxes paid to taxing authorities	(231)	(15)
Noncash investing activities:
Increase (decrease) in asset retirement costs	4	4
Asset retirement obligations assumed by buyer	1	0
Capital Expenditures Incurred but Not yet Paid	$ 279	$ 231