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Pension and Other Postretirement Plans - Allocation of Plan Assets (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	3.00%	3.00%
Deposit Administration Fund | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Percent of total	100.00%
United Airlines, Inc. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	10.00%
United Airlines, Inc. | Equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	30.00%
United Airlines, Inc. | Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	45.00%
United Airlines, Inc. | Fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	4.00%
United Airlines, Inc. | Fixed-income securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	35.00%
United Airlines, Inc. | Fixed-income securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	50.00%
United Airlines, Inc. | Alternatives
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	7.00%
United Airlines, Inc. | Alternatives | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	15.00%
United Airlines, Inc. | Alternatives | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	25.00%

X

- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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+ References

No definition available.

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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