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Revenues - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
Revenues - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Capitalized costs	$ 307		$ 307		$ 295
Contract Balances, Excluding Loyalty Program
Disaggregation of Revenue [Line Items]
Increase (decrease) in contract liabilities			74
Contract balances	759		759		685
Credit Card Branding Agreements
Disaggregation of Revenue [Line Items]
Increase (decrease) in contract liabilities			79
Loyalty Programs
Disaggregation of Revenue [Line Items]
Increase (decrease) in contract liabilities			551
Contract balances	5,491		5,491		$ 4,940
Revenue recognized	$ 533	$ 431	$ 956	$ 855