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Revenue Recognition - Customer Receivables and Contract Balances (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Jan. 01, 2018
Revenue from Contract with Customer [Abstract]    
Customer receivables $ 518 $ 631
Contract liabilities 207 238
Contract assets 64 68
Gross customer receivables 554 669
Allowance for doubtful accounts $ 36 $ 38