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Revenue Recognition - Contract with Customer, Asset and Liability (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Revenue from Contract with Customer [Abstract]    
Customer receivables $ 1,805 $ 1,889
Contract assets 103 108
Contract liabilities 883 950
Accounts receivable, gross 2,000 2,100
Allowance for doubtful accounts receivable $ 148 $ 174