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Notes Receivable (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable, Gross, Allowance, and Net [Abstract]    
Notes receivable $ 16 $ 34
Notes receivable, current 14 18
Notes receivable, noncurrent $ 2 $ 16