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Revenues - Significant Movements in Contract Assets (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Revenue from Contract with Customer [Abstract]    
Beginning balance $ 315 $ 334
Revenues recognized during the period but not billed 298 281
Amounts reclassified to trade accounts receivable 264 282
Ending balance $ 349 $ 333