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Debt - Details of Pass Through Trusts (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Debt Instrument [Line Items] |  |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | $ 1,332 | $ 2,119
United Airlines, Inc. |  |
Debt Instrument [Line Items] |  |
Principal | 1,161 |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | 1,332 | $ 2,119
United Airlines, Inc. | Class AA Pass Through Certificates | February 2018 |  |
Debt Instrument [Line Items] |  |
Principal | $ 677 |
Stated interest rate | 3.50% |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | $ 677 |
United Airlines, Inc. | Class A Pass Through Certificates | February 2018 |  |
Debt Instrument [Line Items] |  |
Principal | $ 258 |
Stated interest rate | 3.70% |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | $ 258 |
United Airlines, Inc. | Class B Pass Through Certificates | May 2018 |  |
Debt Instrument [Line Items] |  |
Principal | $ 226 |
Stated interest rate | 4.60% |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | $ 226 |
United Airlines, Inc. | Pass Through Certificates |  |
Debt Instrument [Line Items] |  |
Total proceeds received from issuance of debt during 2018 and recorded as debt as of September 30, 2018 | $ 1,161 |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585 + Details
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