Combined Notes to the Financial Statements (Unaudited) (Details)			3 Months Ended			6 Months Ended
	Apr. 29, 2021 USD ($)	Jan. 15, 2021 USD ($)	Sep. 30, 2021	Jun. 30, 2021 USD ($) commitment	Mar. 31, 2021 employee	Jun. 30, 2021 USD ($) commitment employee	Jun. 30, 2020 USD ($)	Jun. 27, 2021 aircraft	Apr. 21, 2021 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Percent of capacity cut				46.00%
Proceeds from issuance of debt, net of discounts and fees						$ 11,116,000,000	$ 4,371,000,000
Proceeds from the issuance of common stock						500,000,000
Forecast
Unusual or Infrequent Item, or Both [Line Items]
Percent of capacity cut			26.00%
United Airlines, Inc.
Unusual or Infrequent Item, or Both [Line Items]
Proceeds from issuance of debt, net of discounts and fees						$ 11,116,000,000	$ 4,371,000,000
Boeing 737 MAX 10
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft committed to purchase | aircraft								150
Boeing 737 MAX 8
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft committed to purchase | aircraft								50
Airbus A321neo
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft committed to purchase				70		70		70
B-737-Max
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft committed to purchase | aircraft								200
PSP2
Unusual or Infrequent Item, or Both [Line Items]
Total funding provided under the Payroll Support Program		$ 3,000,000,000.0
Funding provided through direct grants under the Payroll Support Program		2,100,000,000
PSP3
Unusual or Infrequent Item, or Both [Line Items]
Total funding provided under the Payroll Support Program	$ 2,800,000,000
Funding provided through direct grants under the Payroll Support Program	2,000,000,000.0
Voluntary Separation Leave Programs | Employee Separation
Unusual or Infrequent Item, or Both [Line Items]
Number of employees electing to voluntarily separate from the company | employee					4,500	4,500
Unsecured Debt | PSP2 Note
Unusual or Infrequent Item, or Both [Line Items]
Face amount				$ 870,000,000		$ 870,000,000
Term of debt						10 years
Unsecured Debt | PSP2 Note | PSP2
Unusual or Infrequent Item, or Both [Line Items]
Funding provided through loans under the Payroll Support Program		$ 870,000,000
Term of debt		10 years
Unsecured Debt | PSP3 Note
Unusual or Infrequent Item, or Both [Line Items]
Face amount				810,000,000		$ 810,000,000
Term of debt						10 years
Unsecured Debt | PSP3 Note | PSP3
Unusual or Infrequent Item, or Both [Line Items]
Funding provided through loans under the Payroll Support Program	$ 810,000,000
Term of debt	10 years
Secured Debt | 2026 Notes and 2029 Notes
Unusual or Infrequent Item, or Both [Line Items]
Face amount				4,000,000,000.0		$ 4,000,000,000.0			$ 4,000,000,000.0
Secured Debt | 2026 Notes
Unusual or Infrequent Item, or Both [Line Items]
Face amount				$ 2,000,000,000.0		$ 2,000,000,000.0			$ 2,000,000,000.0
Stated interest rate				4.375%		4.375%			4.375%
Secured Debt | 2029 Notes
Unusual or Infrequent Item, or Both [Line Items]
Face amount				$ 2,000,000,000.0		$ 2,000,000,000.0			$ 2,000,000,000.0
Stated interest rate				4.625%		4.625%			4.625%
Secured Debt | Term Loan B Facility due 2028
Unusual or Infrequent Item, or Both [Line Items]
Face amount				$ 5,000,000,000.0		$ 5,000,000,000.0			$ 5,000,000,000.0
Secured Debt | Senior Secured Revolving Credit Facility due 2025 | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity				1,750,000,000		1,750,000,000			1,750,000,000
Secured Debt | Term Loan Facility
Unusual or Infrequent Item, or Both [Line Items]
Face amount				1,400,000,000		1,400,000,000			1,400,000,000
Secured Debt | Credit Agreement | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Face amount				1,000,000,000.0		1,000,000,000.0			1,000,000,000.0
Secured Debt | CARES Act Term Loan Facility
Unusual or Infrequent Item, or Both [Line Items]
Face amount				520,000,000		520,000,000			$ 520,000,000
Pass-Through Certificates | Class A Pass-Through Certificates Issued February 2021 | United Airlines, Inc.
Unusual or Infrequent Item, or Both [Line Items]
Face amount				$ 600,000,000		$ 600,000,000
Stated interest rate				4.88%		4.88%
Proceeds from issuance of debt, net of discounts and fees						$ 600,000,000