Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017	Dec. 31, 2016
Non-cash Investing and Financing Activities [Abstract]
Declared but unpaid dividends	$ 70	$ 60
Issuance of stock upon vesting of restricted stock units	66	38
Cash and Cash Equivalents	937		$ 1,335
Restricted Cash Included in Other Current Assets	17		24
Restricted Cash Included in Other Assets	2		2
Cash, Cash Equivalents and Restricted Cash	$ 956	$ 629	$ 1,361	$ 811
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsCurrent
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent