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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of Accounting Policies [Line Items]      
Capitalized interest costs, including AFUDC $ 2 $ 2 $ 2
Provision for doubtful accounts 13 7 19
Payments related to tax withholding for share-based compensation $ 1 $ 1 $ 1