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Schedule of Information Related to Amounts Recorded Related to Frequent Flyer Program (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Significant Accounting Policies [Line Items] |  |  |  |
Cash Proceeds from Miles Sold and Earned |  | $ 2,343 | $ 3,022 | $ 2,999
Other Revenue Recognized Upon Award of Miles to Third-Party Customers | [1] | 1,183 | 1,221 | 1,050
Increase in Frequent Flyer Deferred Revenue for Miles Awarded | [2] | 2,025 | 2,050 | 2,173
Decrease in Advanced Purchase of Miles | [3] | $ (865) | $ (249) | $ (224)
[1] | This amount represents other revenue recognized during the period from the sale of miles to third parties, representing the marketing-related deliverable services component of the sale.
[2] | This amount represents the increase to Frequent flyer deferred revenue during the period.
[3] | This amount represents the net decrease in the advance purchase of miles obligation due to cash payments for the sale of miles less than miles awarded to customers.

X
- Definition Cash proceeds from miles sold and earned. + References No definition available. + Details
Name: | ual_CashProceedsFromMilesSoldAndEarned
Namespace Prefix: | ual_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Net increase in advanced purchase of miles in frequent flyer programs. + References No definition available. + Details
Name: | ual_NetIncreaseInAdvancedPurchaseOfMilesInFrequentFlyerPrograms
Namespace Prefix: | ual_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Other revenue recognized upon award of miles to third party customers in frequent flyer program. + References No definition available. + Details
Name: | ual_OtherRevenueRecognizedUponAwardOfMilesToThirdPartyCustomersInFrequentFlyerProgram
Namespace Prefix: | ual_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | ual_SignificantAccountingPoliciesLineItems
Namespace Prefix: | ual_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in the amount of unearned revenue related to tickets sold but not yet recognized as revenue and the estimated incremental cost for points or miles outstanding and awards that expect to be redeemed through customer loyalty programs. + References No definition available. + Details
Name: | us-gaap_IncreaseDecreaseInDeferredAirTrafficRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration