Revenue Recognition - Contract Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018		Sep. 30, 2018		Jun. 30, 2018	Dec. 31, 2017	[3]
Contract with Customer, Liability	$ 160	[1],[2]	$ 160	[1],[2]	$ 152	$ 131
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized	(160)	[4]	(506)	[5]
Contract with Customer, Liability, Current	168		535
Contract with Customer, Liability, Noncurrent	6		6
Customer Loyalty Program [Member]
Contract with Customer, Liability	72	[1],[2]	72	[1],[2]	68	62
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized	(34)	[4]	(97)	[5]
Contract with Customer, Liability, Current	38		107
Contract with Customer, Liability, Revenue Recognized	5		29
Customer Advances [Member]
Contract with Customer, Liability	88	[1],[2]	88	[1],[2]	$ 84	$ 69
Contract with Customer, Liability, Change in Timeframe, Performance Obligation Satisfied, Revenue Recognized	(126)	[4]	(409)	[5]
Contract with Customer, Liability, Current	130		428
Contract with Customer, Liability, Revenue Recognized	$ 5		$ 64

[1] (3) $6 million included within Deferred credits and other liabilities.

[2] (In millions) Total Rewards   Customer Advances   Total Beginning balance as of December 31, 2017 ⁽¹⁾ $ 62
  $ 69
  $ 131
Amount recognized during the period ⁽²⁾ (97 )   (409 )   (506 ) Amount accrued during the period 107
  428
  535
Ending balance as of September 30, 2018 ⁽³⁾ $ 72
  $ 88
  $ 160

[3] (1) $2 million included within Deferred credits and other liabilities.

[4] (1) Includes $5 million for Total Rewards and $5 million for Customer Advances recognized from the June 30, 2018 Contract liability balances.

[5] (2) Includes $29 million for Total Rewards and $64 million for Customer Advances recognized from the December 31, 2017 Contract liability balances.