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