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Components of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Beginning Balance, deferred taxes |  | $ 26 |  | $ (256) | $ (116)
Other comprehensive income (loss) before reclassifications, deferred taxes |  | 74 |  | 187 | 88
Amounts reclassified from accumulated other comprehensive income, deferred taxes |  | (21) |  | 95 | (228)
Reclassification of stranded tax effects | [1] | (118) |  |  |
Net other comprehensive income (loss), deferred taxes |  | (65) |  | 282 | (140)
Ending Balance, deferred taxes |  | (39) | [2] | 26 | (256)
Beginning Balance |  | 8,659 |  | 8,966 | 2,396
Other comprehensive income (loss) before reclassifications, net of tax |  | (239) |  | (334) | (159)
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 39 |  | 336 | 407
Total other comprehensive income (loss), net |  | (318) |  | 2 | 248
Ending Balance |  | 8,806 |  | 8,659 | 8,966
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Beginning Balance |  | (829) |  | (831) | (1,079)
Reclassification of stranded tax effects |  | (118) |  |  |
Ending Balance |  | (1,147) |  | (829) | (831)
Pension and Other Postretirement Liabilities |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Beginning Balance, before tax |  | (854) |  | (363) | (472)
Other comprehensive income (loss) before reclassifications, before tax | [3] | (306) |  | (517) | 78
Amounts reclassified from accumulated other comprehensive income, before tax |  | 58 |  | 26 | 31
Net other comprehensive income (loss), before tax |  | (248) |  | (491) | 109
Ending Balance, before tax |  | (1,102) |  | (854) | (363)
Derivative Contracts | Fuel |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Beginning Balance, before tax |  | (2) |  | (215) | (499)
Other comprehensive income (loss) before reclassifications, before tax |  |  |  | (4) | (320)
Amounts reclassified from accumulated other comprehensive income, before tax |  | 2 |  | 217 | 604
Net other comprehensive income (loss), before tax |  | 2 |  | 213 | 284
Ending Balance, before tax |  |  |  | (2) | (215)
Investments and Other |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Beginning Balance, before tax |  | 1 |  | 3 | 8
Other comprehensive income (loss) before reclassifications, before tax |  | (7) |  |  | (5)
Amounts reclassified from accumulated other comprehensive income, before tax |  |  |  | (2) |
Net other comprehensive income (loss), before tax |  | (7) |  | (2) | (5)
Ending Balance, before tax |  | $ (6) |  | $ 1 | $ 3
[1] | This amount represents the reclassification from AOCI to RE of the stranded tax effects resulting from the enactment of the Tax Act.
[2] | Deferred tax balance relates mainly to Pension and Other Postretirement Liabilities.
[3] | Prior service credits increased by $0 million, $30 million and $0 million and actuarial losses increased by approximately $306 million, $560 million and $78 million for 2017, 2016 and 2015, respectively.

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- Definition Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, attributable to parent. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners). + References No definition available. + Details
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