Revenue Recognition - Frequent Flyer Liability (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Frequent Flyer Liability Activity [Roll Forward]
Mileage credits earned	$ 1,550	$ 1,459
Travel mileage credits redeemed	(1,298)	(1,204)
Non-travel mileage credits redeemed	(82)	(81)
Frequent Flyer
Frequent Flyer Liability Activity [Roll Forward]
Deferred revenue (current and noncurrent), beginning	6,321	5,922
Deferred revenue (current and noncurrent), ending	$ 6,491	$ 6,096

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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant