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Income Taxes (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 35.00%    
DEFERRED TAX LIABILITIES:      
Accelerated depreciation $ 3,193 $ 4,726  
Other 86 134  
Deferred Tax Liabilities, Gross 3,279 4,860  
DEFERRED TAX ASSETS:      
Fuel derivative instruments 12 233  
Deferred tax assets construction obligation 326 402  
Accrued employee benefits 309 451  
Other 274 400  
Total deferred tax assets 921 1,486  
Total deferred tax liabilities 2,358 3,374  
CURRENT:      
Federal 904 778 $ 1,292
State 72 69 114
Total current 976 847 1,406
DEFERRED:      
Federal 192 426 (97)
State 5 30 (11)
Change in federal statutory rate (1,410) 0 0
Total deferred (1,213) 456 (108)
Total (237) 1,303 1,298
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]      
Tax at statutory U.S. tax rates 1,138 1,241 1,218
State income taxes, net of federal benefit 50 64 66
Change in federal statutory rate (1,410) 0 0
Other, net $ (15) $ (2) $ 14