Accounting Policies - Accounts Receivable, Net and Other (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 16,887	$ 16,677
Customer Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	10,600	9,400
Vendor Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	2,500	3,200
Seller Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 790	$ 710