Revenues - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Capitalized costs	$ 313	$ 295
Contract Balances, Excluding Loyalty Program
Disaggregation of Revenue [Line Items]
Increase (decrease) in contract liabilities	81
Contract balances	766	685
Loyalty Program
Disaggregation of Revenue [Line Items]
Increase (decrease) in contract liabilities	682
Contract balances	5,622	$ 4,940
Revenue recognized	$ 1,315

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

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