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Income Taxes (Details 2) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Income taxes    
Valuation allowance $ 380 $ 409
Deferred tax asset valuation allowance adjustment 29  
Federal    
Income taxes    
Tax credit carryforwards 28  
Investment tax credits | State    
Income taxes    
Tax credit carryforwards 36  
Tax credit carryforwards, net of federal income tax 23  
Alternative minimum tax credits    
Income taxes    
Tax credit carryforwards $ 79