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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation, Revision of Estimate   $ 0 $ 5 $ 30
Asset Retirement Obligation, Accretion Expense   7 7 7
Asset Retirement Obligation, Liabilities Incurred   2 $ 1 2
Litigation Settlement, Amount Awarded from Other Party   85    
Proceeds from Decommissioning Trust Fund Assets   53    
Loss Contingency, Damages Awarded, Value $ 50      
Customer Refund Liability, Current   3    
Asset Retirement Obligation, Period Increase (Decrease)   6    
Asset Retirement Obligations, Significant Changes       $ 17
Asset Retirement Obligation Rate Recovery related to Trojan Plant   4    
Utility Plant [Domain]        
Asset Retirement Obligation, Revision of Estimate   2    
Asset Retirement Obligation, Accretion Expense   7    
Asset Retirement Obligation, Liabilities Incurred   $ 3