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Schedule II - Valuation and Qualifying Accounts (Details) - UAL and United - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Allowance for doubtful accounts |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | $ 8 | $ 7 | $ 10
Additions Charged to Costs and Expenses | 17 | 17 | 20
Deductions | 16 | 16 | 23
Other | 0 | 0 | 0
Balance at End of Period | 9 | 8 | 7
Obsolescence allowance-spare parts |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | 412 | 354 | 295
Additions Charged to Costs and Expenses | 76 | 73 | 75
Deductions | 63 | 15 | 17
Other | 0 | 0 | 1
Balance at End of Period | 425 | 412 | 354
Valuation allowance for deferred tax assets |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | 59 | 63 | 68
Additions Charged to Costs and Expenses | 0 | 2 | 11
Deductions | 1 | 6 | 27
Other | 0 | 0 | 11
Balance at End of Period | $ 58 | $ 59 | $ 63

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