Description of Business and Accounting Policies - Accounts Receivable, Net and Other (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 13,164	$ 8,339
Allowance for doubtful accounts	348	237	$ 189
Additions to allowance for doubtful accounts	626	451	289
Deductions to allowance for doubtful accounts	515	403	$ 290
Customer receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	6,400	3,900
Seller receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	692	661
Vendor receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net and other	$ 2,600	$ 2,000