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Gallatin Coal Combustion Residual Facilities (Details) - USD ($)
 9 Months Ended
Jun. 30, 2018
Sep. 30, 2017
Other Long-Term Liabilities    
Current regulatory assets $ 421,000,000 $ 447,000,000
Gallatin coal combustion residual facilities original estimated cost for dewatering facility 91,000,000  
Gallatin coal combustion residual facilities estimated cost relocate offsite 2,000,000,000  
Gallatin coal combustion residual facilities estimated cost to cap and close 200,000,000  
TDEC civil penalties related to Gallatin 17,000  
Plaintiff civil penalties related to Gallatin 37,500  
Gallatin coal combustion residual facilities estimated cost relocate onsite 900,000,000  
Other long-term liabilities    
Other Long-Term Liabilities    
Gallatin coal combustion residual facilities 875,000,000 880,000,000
Accounts payable and accrued liabilities    
Other Long-Term Liabilities    
Gallatin coal combustion residual facilities 20,000,000  
Removal Costs    
Other Long-Term Liabilities    
Current regulatory assets $ 34,000,000 $ 0