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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales        
Allowance for Doubtful Accounts Receivable, Current $ 0 $ 1    
MLGW prepayment       $ 1,500
MLGW prepayment period       180 months
Recognition of Deferred Revenue 100 100 $ 100  
MLGW        
Energy Prepayment Obligations and Discounts on Sales        
Recognition of Deferred Revenue $ 1,490      
Deferred revenue expected recognition each year 100      
Deferred revenue, revenue recognized $ 10 100    
Total        
Energy Prepayment Obligations and Discounts on Sales        
Discounts reducing electricity sales $ 46 $ 46