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Income Taxes Deferred Tax Asset Expiration Periods (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Income Tax Examination [Line Items]    
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax $ 178 $ 175
Expires from 2018 - 2037 [Member] | Domestic Tax Authority [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration 3,400  
Expires from 2018 - 2037 [Member] | State and Local Jurisdiction [Member] | Domestic Tax Authority [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration 3,200  
Expires from 2029 - 2036 [Member] | Foreign Tax Authority [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Operating Loss Carryforwards, Foreign 134  
Does Not Expire [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax 8  
Expires 2018 - 2027 [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Tax Credit Carryforwards 147  
Majority expire from 2018 - 2023 [Member]    
Income Tax Examination [Line Items]    
Deferred Tax Assets, Tax Credit Carryforwards, Foreign $ 21