Revenue Recognition - Narrative (Details) - USD ($) $ in Billions	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Revenue from Contract with Customer [Abstract]
Revenue recognized that was previously deferred	$ 1.8	$ 3.0
Cash sales of mileage credits	$ 2.9	$ 2.2
Redemption period for majority of new miles (in years)	2 years

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration