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Income Taxes Deferred Tax Assets and Valuation Allowances (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount $ (13)  
Deferred Tax Assets, Operating Loss Carryforwards 935 $ 1,128
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax 178 175
Deferred Tax Assets, Capital Loss Carryforwards   4
Proceeds from Income Tax Refunds 144  
Capital Loss Carryforward [Member]    
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount (4)  
Foreign Tax Authority [Member]    
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount (5)  
Annual Rate Reductions [Member]    
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount (24)  
Valuation Allowance, Operating Loss Carryforwards [Member]    
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount 18  
Valuation Allowance, Tax Credit Carryforward [Member]    
Valuation Allowance [Line Items]    
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount 2  
Valuation Allowance of Deferred Tax Assets [Member]    
Valuation Allowance [Line Items]    
Valuation Allowances and Reserves, Balance 133 $ 123
Deferred Income Tax Charge [Member]    
Valuation Allowance [Line Items]    
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax Increase (Decrease) (143)  
Deferred Compensation, Share-based Payments [Member]    
Valuation Allowance [Line Items]    
Deferred Tax Assets, Operating Loss Carryforwards $ 8