Cash Flow Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Increase in PP&E related to increase in asset retirement obligations		$ (37)	$ (1,017)	$ 402
Interest Paid, Net		1,163	1,151	920
Income Taxes Paid, Net	[1]	1,168	(318)	523
Payments for (Proceeds from) Short-term Investments [Abstract]
Short-term investments purchased		(6,617)	(1,753)
Short-term investments sold		4,827	1,702
Net Sales (Purchases) of Short-Term Investments		$ 1,790	$ 51	$ 0

[1] Net of $585 million and $642 million in 2016 and 2015, respectively, related to refunds received rom the Internal Revenue Service.