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Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	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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Refundable Tickets Expiration Period

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Summary Of Significant Accounting Policies [Line Items]

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Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings tax effect.

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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

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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

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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.

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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

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-Section 50
-Paragraph 1
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Stated as a percentage, the estimated or actual value of the asset at the end of its useful life or when it is no longer serviceable (cannot be used for its original purpose) divided by its [historical] capitalized cost.

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