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Other Material Contingencies Other Material Contingencies - Other (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Guarantor Obligations [Line Items]    
Guarantee obligations maximum exposure $ 2,193 $ 2,370
Financial Guarantee for Indemnity Agreements for Surety Bonds    
Guarantor Obligations [Line Items]    
Guarantee obligations maximum exposure 70  
Certain Obligations Under Agreement to Sell the Retail Electric Supply Business    
Guarantor Obligations [Line Items]    
Guarantee obligations maximum exposure 53  
Financial Guarantee to Public Utility Commission of Texas    
Guarantor Obligations [Line Items]    
Guarantee obligations maximum exposure $ 5