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Revenues and Contract Costs - Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jan. 01, 2021	Dec. 31, 2020	Jan. 01, 2020
Disaggregation of Revenue [Line Items]
Contract asset	$ 878	$ 923	$ 878	$ 923	$ 937	$ 937	$ 1,150
Contract liability	6,034	5,321	6,034	5,321	$ 5,598	5,598	$ 5,307
Revenue recognized related to contract liabilities existing	(161)	$ (124)	4,200	$ (4,200)
Prepaid expenses and other
Disaggregation of Revenue [Line Items]
Contract asset	701		701			733
Other assets
Disaggregation of Revenue [Line Items]
Contract asset	177		177			204
Other current liabilities
Disaggregation of Revenue [Line Items]
Contract liability	5,159		5,159			4,843
Other liabilities
Disaggregation of Revenue [Line Items]
Contract liability	$ 875		$ 875			$ 755