Accounting Policies - Accounts Receivable, Net and Other (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 12,607	$ 13,164
Allowance for Doubtful Accounts Receivable, Current	403	348
Customer Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	6,900	6,400
Vendor Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	2,200	2,600
Seller Receivables, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 640	$ 692