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Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Valuation allowance for deferred tax assets |  |  |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |  |
Balance at Beginning of Period |  | $ 48 | $ 4,751 | $ 4,591
Additions Charged to Costs and Expenses |  | 47 |  | 156
Deductions | [1] | 27 | 4,703 |
Other |  |  |  | 4
Balance at End of Period |  | 68 | 48 | 4,751
UAL and United | Allowance for doubtful accounts |  |  |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |  |
Balance at Beginning of Period |  | 18 | 22 | 13
Additions Charged to Costs and Expenses |  | 18 | 25 | 45
Deductions | [1] | 26 | 29 | 36
Balance at End of Period |  | 10 | 18 | 22
UAL and United | Obsolescence allowance-spare parts |  |  |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |  |
Balance at Beginning of Period |  | 235 | 169 | 162
Additions Charged to Costs and Expenses |  | 61 | 38 | 35
Deductions | [1] | 16 |  | 28
Other |  | 15 | 28 |
Balance at End of Period |  | 295 | 235 | 169
United Airlines, Inc. | Valuation allowance for deferred tax assets |  |  |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |  |
Balance at Beginning of Period |  | 48 | 4,721 | 4,561
Additions Charged to Costs and Expenses |  | 47 |  | 167
Deductions | [1] | 27 | 4,673 |
Other |  |  |  | (7)
Balance at End of Period |  | $ 68 | $ 48 | $ 4,721
[1] | Deduction from reserve for purpose for which reserve was created.

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- Definition Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges to costs and expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges to accounts other than costs and expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Total of the deductions in a given period to allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, representing receivables written off as uncollectible and portions of the reserves utilized, respectively. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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