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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Counterparty Credit Risk (Details)
 12 Months Ended
Sep. 30, 2013
Customers
Sep. 30, 2012
Customers
Derivative    
Number of active future commission merchants 2  
Credit of Customers
    
Derivative    
Number of customers that represent the percent of outstanding accounts receivable 3 3
Percent of total outstanding accounts receivables by customers 27.00% 26.00%
Percent of operating revenue represented by largest industrial customer 3.00% 5.00%
Power Purchase Agreement
    
Derivative    
Megawatts in a power purchase agreement that expires on March 31, 2032 440