Sales of Certain Receivables - Sales of Service Receivables (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Other current assets	$ 1,642	$ 2,496
Other current liabilities	877	810
Variable Interest Entity, Not Primary Beneficiary | Factoring Arrangement
Variable Interest Entity [Line Items]
Revolving receivables facility, maximum borrowing capacity	950
Revolving receivables facility, outstanding borrowings	775	772
Other current assets	202	378
Other current liabilities	$ 313	$ 357