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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Taxes [Line Items] |  |  |  |  |  |
Federal statutory income tax rate |  |  |  | 35.00% |  | 35.00%
Deferred income tax expense |  |  |  | $ 945 | $ 1,648 | $ (3,177)
Income tax benefit |  |  |  | 192 |  |
Provisional one-time transition tax on previously deferred foreign earnings |  |  |  | 19 |  |
Undistributed cumulative earnings of foreign subsidiaries contributing to one-time transition tax | $ 122 |  |  | 122 |  |
Income tax adjustments | (192) | $ 180 |  | (192) | 180 | (3,130)
Valuation allowance | 63 | 68 |  | 63 | 68 |
Unrecognized tax benefits | 21 | $ 74 |  | 21 | 74 | 24
Unrecognized tax benefits that would impact effective tax rate if recognized | 21 |  |  | $ 21 |  |
Scenario Forecast |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Federal statutory income tax rate |  |  | 21.00% |  |  |
Minimum |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
State tax benefits of the NOL carry forwards expiration period, years |  |  |  | 5 years |  |
Maximum |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
State tax benefits of the NOL carry forwards expiration period, years |  |  |  | 20 years |  |
2026 |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Net operating loss expiration amount | 200 |  |  | $ 200 |  |
2029 |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Net operating loss expiration amount | 500 |  |  | 500 |  |
Thereafter |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Net operating loss expiration amount | 1,700 |  |  | 1,700 |  |
Fuel | Designated as Hedging Instrument |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Deferred income tax expense |  |  |  |  | $ 180 | $ 180
Federal |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
NOL carry forwards | 2,400 |  |  | 2,400 |  |
State |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
NOL carry forwards | 49 |  |  | 49 |  |
NOL carry forwards, net of valuation allowance | 52 |  |  | 52 |  |
Valuation allowance | $ 63 |  |  | $ 63 |  |

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- Definition Amount of deferred foreign income tax expense (benefit) pertaining to income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 + Details
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