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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) (USD $)
 12 Months Ended 24 Months Ended 108 Months Ended 12 Months Ended 108 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2004
Sep. 30, 2004
Sep. 30, 2013
DEU [Member]
Sep. 30, 2013
MLGW
Sep. 30, 2012
MLGW
Sep. 30, 2011
MLGW
Sep. 30, 2013
MLGW
Sep. 30, 2013
Total
Sep. 30, 2012
Total
Sep. 30, 2011
Total
Energy Prepayment Obligations and Discounts on Sales                          
Incremental amount of discounted energy units $ 1,000,000                        
Discount per kilowatt-hour 0.025                        
Discounted energy units contract period 1 5 years                        
Discounted energy units contract period 2 10 years                        
Discounted energy units contract period 3 15 years                        
Discounted energy units contract period 4 20 years                        
Total sales through DEU program since inception         55,000,000                
Deferred revenue, revenue recognized 2,000,000 5,000,000 5,000,000     54,000,000 100,000,000 100,000,000 100,000,000 990,000,000 47,000,000 47,000,000 47,000,000
MLGW prepayment       1,500,000,000                  
MLGW prepayment period       180 months                  
Deferred revenue expected recognition each year             $ 100,000,000