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Income Taxes (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
DEFERRED TAX LIABILITIES:      
Accelerated depreciation $ 4,726 $ 4,429  
Other 134 62  
Deferred Tax Liabilities, Gross 4,860 4,491  
DEFERRED TAX ASSETS:      
Fuel derivative instruments 233 750  
Deferred tax assets construction obligation 402 289  
Accrued employee benefits 451 541  
Other 400 421  
Total deferred tax assets 1,486 2,001  
Total deferred tax liabilities 3,374 2,490  
CURRENT:      
Federal 778 1,292 $ 203
State 69 114 29
Total current 847 1,406 232
DEFERRED:      
Federal 426 (97) 421
State 30 (11) 27
Total deferred 456 (108) 448
Total 1,303 1,298 680
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]      
Tax at statutory U.S. tax rates 1,241 1,218 636
State income taxes, net of federal benefit 64 66 37
Other, net $ (2) $ 14 $ 7