Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Trade receivables (included in accounts receivable, net)			$ 7,698	$ 6,717
Contract liabilities (included in accrued expenses)	$ 85	$ 75	$ 85	$ 73
Change in Contract with Customer, Liability [Roll Forward]
Contract liabilities (included in accrued expenses) beginning balance	73	67
Rewards earnings and gift card issuances	99	90
Redemption and breakage	(87)	(82)
Contract liabilities (included in accrued expenses) ending balance	$ 85	$ 75