REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract Liabilities Activity (Details) - Mileage Plan Revenue [Member] - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Change in Contract with Customer, Liability [Roll Forward]
Total Deferred Revenue balance at January 1	$ 1,874	$ 1,725
Increase in liability for mileage credits issued	451	402
Total Deferred Revenue balance at June 30	1,949	1,777
Passenger revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	(325)	(306)
Mileage plan other revenue [Member]
Change in Contract with Customer, Liability [Roll Forward]
Revenue recognized	$ (51)	$ (44)

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Name:	us-gaap_ContractWithCustomerLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant