Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Trade receivables (included in accounts receivable, net)			$ 6,848	$ 6,717
Contract liabilities (included in accrued expenses)	$ 78	$ 76	$ 78	$ 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	179	181
Redemption and breakage	(174)	(172)
Contract liabilities (included in accrued expenses) ending balance	$ 78	$ 76