Supplementary Information for Certain Balance Sheet Accounts - Prepaid and Other Current Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Prepayments and accrued income	$ 164	$ 132
Deferred contract costs	69
Derivatives and investments	21	29
Deferred compensation plan assets	17	21
Retention incentives	7	7
Corporate income and other taxes	201	170
Other current assets	68	71
Total prepaid and other current assets	$ 547	$ 430