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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.