Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Counterparty Credit Risk (Details) (USD $) In Millions, unless otherwise specified						12 Months Ended
	Sep. 30, 2012 merchants	Sep. 27, 2012	Sep. 30, 2011 Customers	Sep. 30, 2012 MF Global Holding Ltd.	Oct. 31, 2011 MF Global Holding Ltd.	Sep. 30, 2012 Credit of Customers Customers	Sep. 30, 2011 Credit of Customers	Sep. 30, 2012 Credit of Suppliers Suppliers	Sep. 30, 2012 Power Purchase Agreement Megawatts
Derivative
Number of customers that represent the 26 percent of outstanding accounts receivable			3			3
Percent of total outstanding accounts receivables by three customers			26.00%			26.00%
Percent of operating revenue represented by largest industrial customer						5.00%	4.00%
Number of active future commission merchants	2
Collateral already posted					$ 33
Amount already recovered				8
Amount of write-off		$ 2
Number of suppliers with coal contracts								13
Megawatts in a power purchase agreement that expires on March 31, 2032									440