Accounting Policies - Contract Balances and Activity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies
Receivables (included in accounts receivable, net)		$ 7,258	$ 7,873
Contract liabilities (included in accrued expenses)	$ 53	$ 63	$ 53
Rollforward of contract liabilities
Balance at beginning of period	53
Adoption of ASU 2014-09	17
Loyalty program earnings and gift card issuance	236
Redemption and breakage	(243)
Balance at end of period	$ 63