Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Trade receivables (included in accounts receivable, net)			$ 6,717	$ 6,497
Contract liabilities (included in accrued expenses)	$ 67	$ 53	$ 73	$ 67
Change in Contract with Customer, Liability [Abstract]
Balance at December 31, 2018	67	53
Adoption of ASU 2014-09	0	17
Rewards earnings and gift card issuances	365	332
Redemption and breakage	(359)	(335)
Balance at December 31, 2019	$ 73	$ 67