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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Income Taxes [Line Items] |  |  |  |  |  |
Federal statutory income tax rate |  |  |  |  | 35.00% |
Deferred income tax expense |  |  |  | $ 1,648 | $ (3,177) | $ 13
Alternative minimum tax ("AMT") credit carryforwards, net of valuation allowance | $ 108 |  |  | 108 |  |
Alternative minimum tax ("AMT") credit carryforwards, valuation allowance | 8 |  |  | 8 |  |
Income tax adjustments | 180 | $ 88 | $ (3,218) | 180 | (3,130) |
Valuation allowance | 68 | 48 |  | 68 | 48 |
Unrecognized tax benefits | 74 | 24 |  | 74 | 24 | $ 9
Unrecognized tax benefits that would impact effective tax rate if recognized | 20 |  |  | $ 20 |  |
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 | 500 |  |  | $ 500 |  |
2027 |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Net operating loss expiration amount | 1,400 |  |  | 1,400 |  |
Thereafter |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Net operating loss expiration amount | 2,500 |  |  | 2,500 |  |
Fuel | Designated as Hedging Instrument |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Deferred income tax expense |  |  |  | 180 | 180 |
State |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
NOL carry forwards | 56 |  |  | 56 |  |
NOL carry forwards, net of valuation allowance | 36 |  |  | 36 |  |
Valuation allowance |  | $ 68 |  |  | $ 68 |
Federal |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
NOL carry forwards | $ 4,400 |  |  | $ 4,400 |  |

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- Definition Income tax adjustments that include items such as deferred income tax expense in AOCI and valuation allowance adjustments. + References No definition available. + Details
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- Definition Amount of deferred 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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=63044130 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=65882285&loc=d3e32639-109319 Reference 4: 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 5: 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 + Details
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