Commitments and Contingencies - Narrative (Details) $ / shares in Units, shares in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2021 employee	Jun. 30, 2021 USD ($) aircraft employee	Nov. 30, 2020 USD ($)	Nov. 30, 2018 USD ($) $ / shares shares
Commitments and Contingencies [Line Items]
Number of aircraft delivered | aircraft		11
Remaining term of debt		11 years
Employee Separation | Voluntary Separation Leave Programs
Commitments and Contingencies [Line Items]
Number of employees electing to voluntarily separate from the company | employee	4,500	4,500
Flight Equipment
Commitments and Contingencies [Line Items]
Number of aircraft delivered | aircraft		7
Operating Lease Right-of-Use Assets
Commitments and Contingencies [Line Items]
Number of aircraft delivered | aircraft		4
Current/Long-Term Obligations under Operating Leases
Commitments and Contingencies [Line Items]
Number of aircraft delivered | aircraft		4
United Airlines, Inc.
Commitments and Contingencies [Line Items]
Number of employees | employee		84,400
Percentage of employees represented by various U.S. labor organizations		85.00%
Kingsland
Commitments and Contingencies [Line Items]
Number of shares that may be put to United (in shares) | shares				144.8
Number of ADRs that may be put into United (in shares) | shares				18.1
Market price of common stock at fifth anniversary (in dollars per share) | $ / shares				$ 12
Aggregate maximum possible combined put payment and guarantee amount				$ 217,000,000
Kingsland | Standby Letters of Credit
Commitments and Contingencies [Line Items]
Cash collateral for a standby letter of credit		$ 217,000,000	$ 217,000,000
BRW | Term Loan Receivable
Commitments and Contingencies [Line Items]
Loan amount				$ 456,000,000
Tax-Exempt Special Facilities Revenue Bonds
Commitments and Contingencies [Line Items]
Aggregate principal amount of guarantee		1,900,000,000
Aircraft Mortgage Debt
Commitments and Contingencies [Line Items]
Aggregate principal amount of guarantee		113,000,000
Floating Rate Debt
Commitments and Contingencies [Line Items]
Debt		$ 13,400,000,000
Loans and Leases from Non-U.S. Entities
Commitments and Contingencies [Line Items]
Remaining term of debt		11 years
Aggregate balance		$ 10,200,000,000
Boeing 737-700
Commitments and Contingencies [Line Items]
Number of aircraft committed to purchase | aircraft		6
Airbus A319
Commitments and Contingencies [Line Items]
Number of aircraft committed to purchase | aircraft		11