Accounting Policies and Supplemental Disclosures - Accounts Receivable, Net and Other (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 20,832	$ 20,816
Allowance for doubtful accounts	968	718
Customer receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	13,100	12,600
Vendor receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	3,300	4,200
Seller receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 446	$ 863