For an unclassified balance sheet, amounts due from parties associated with the reporting entity as evidenced by a written promise to pay.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.3)
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Name:	us-gaap_NotesReceivableRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Investments - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2021
CLEAR
Investments [Line Items]
Ownership interest (percent)	6.00%	6.00%
Long-term investments	$ 120	$ 340
Grupo Aeromexico
Investments [Line Items]
Long-term investments		0
LATAM
Investments [Line Items]
Long-term investments		0
Virgin Atlantic
Investments [Line Items]
Long-term investments		0
Grupo Aeromexico, LATAM, and Virgin Atlantic
Investments [Line Items]
Receivable		525
Grupo Aeromexico
Investments [Line Items]
Receivable		185
GOL
Investments [Line Items]
Loan issued	$ 250
Support for investee carriers \| Grupo Aeromexico, LATAM, and Virgin Atlantic
Investments [Line Items]
Notes payable		525
Support for investee carriers \| Grupo Aeromexico
Investments [Line Items]
Notes payable		$ 185