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Schedule II - Valuation and Qualifying Accounts (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Valuation and qualifying accounts      
Federal benefit for the state-related portion of the deferred tax valuation allowance $ 88    
Deferred Tax Valuation Allowance      
Valuation and qualifying accounts      
Charged to Costs and Expenses 294    
Charged to Other Accounts 88    
Balance at End of Period 382    
Other asset valuation allowance      
Valuation and qualifying accounts      
Balance at Beginning of Period 10    
Charged to Costs and Expenses 58 $ 10  
Balance at End of Period 68 10  
Environmental      
Valuation and qualifying accounts      
Balance at Beginning of Period 8 8 $ 11
Charged to Costs and Expenses   1 1
Deductions (1) (1) (4)
Balance at End of Period $ 7 $ 8 $ 8