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Schedule II - Valuation and Qualifying Accounts (Details) - UAL and United - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Allowance for doubtful accounts |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | $ 9 | $ 8 | $ 7
Additions Charged to Costs and Expenses | 70 | 17 | 17
Deductions | 16 | 16 | 16
Other | 15 | 0 | 0
Balance at End of Period | 78 | 9 | 8
Obsolescence allowance-spare parts |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | 425 | 412 | 354
Additions Charged to Costs and Expenses | 88 | 76 | 73
Deductions | 35 | 63 | 15
Other | 0 | 0 | 0
Balance at End of Period | 478 | 425 | 412
Allowance for credit losses - notes receivable |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | 0 |  |
Additions Charged to Costs and Expenses | 518 |  |
Deductions | 0 |  |
Other | 4 |  |
Balance at End of Period | 522 | 0 |
Valuation allowance for deferred tax assets |  |  |
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |
Balance at Beginning of Period | 58 | 59 | 63
Additions Charged to Costs and Expenses | 197 | 0 | 2
Deductions | 8 | 1 | 6
Other | 0 | 0 | 0
Balance at End of Period | $ 247 | $ 58 | $ 59

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