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Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items] |  |  |
Residual value | 10.00% |  |
Carrying value of computer software | $ 345 | $ 356 |
Advertising expense | 217 | 220 | $ 201
ASU 2018-02 |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings, tax effect | $ 118 |  |
Minimum |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Refundable tickets expiration period | 1 year |  |
Computer software |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Depreciation expense related to computer software | $ 117 | $ 108 | $ 93
Aircraft and aircraft spare parts |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Residual value | 10.00% |  |

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- Definition Refundable Tickets Expiration Period + References No definition available. + Details
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- Definition Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings tax effect. + References No definition available. + Details
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- Definition Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 35 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848 + Details
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- Definition Amount before accumulated amortization of capitalized costs for computer software, including but not limited to, acquired and internally developed computer software. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 + Details
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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