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Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2017 | Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Valuation allowance | $ 68 | $ 63 |
Income tax adjustment from AOCI | $ 180 |  |
Valuation allowance for deferred tax assets |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Valuation allowance |  |  | $ 465
Pension and Other Postretirement Liabilities | Valuation allowance for deferred tax assets |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Valuation allowance |  |  | 285
Fuel | Designated as Hedging Instrument | Valuation allowance for deferred tax assets |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Valuation allowance |  |  | $ 180

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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