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Revenue Recognition - Narratives (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Revenue from Contract with Customer [Abstract]          
Allowance for credit losses, period increase (decrease) $ 5   $ 10    
Allowance for credit losses 128   128   $ 138
Provisions for doubtful receivables 48 $ 51 128 $ 204  
Contract assets 339   339   279
Deferred revenue 359   359   279
Contract with Customer, Liability, Current 35   35   21
Contract with Customer, Liability, Noncurrent $ 26   $ 26   $ 26