SUPPLEMENTARY DATA QUARTERLY FINANCIAL DATA - UNAUDITED - Quarterly Financial Data - Unaudited (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues	$ 5,289	$ 5,051	$ 5,409	$ 5,009	$ 5,251	$ 5,078	$ 5,211	$ 4,912		$ 20,758	$ 20,452	$ 15,407
Operating income	422	596	818	530	424	790	744	546		2,366	2,504	1,424
Net income	$ 317	$ 503	$ 667	$ 420	$ 114	$ 485	$ 489	$ 371		$ 1,907	$ 1,459	$ 808
Basic earnings per share (in USD per share)	$ 0.93	$ 1.45	$ 1.89	$ 1.17	$ 0.31	$ 1.30	$ 1.29	$ 0.96		$ 5.45	$ 3.89	$ 2.78
Diluted earnings per share (in USD per share)	$ 0.92	$ 1.43	$ 1.87	$ 1.16	$ 0.31	$ 1.29	$ 1.28	$ 0.95		$ 5.38	$ 3.84	$ 2.73
Effect of Prior Period Adjustment | Loyalty Program Error
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues		$ 2	$ 63	$ 3
Operating income		26	78	30
Net income		$ 20	$ 57	$ 22					$ 99
Basic earnings per share (in USD per share)		$ 0.06	$ 0.16	$ 0.06
Diluted earnings per share (in USD per share)		$ 0.05	$ 0.16	$ 0.07