Risk Management Activities and Derivative Transactions Derivatives Under Financial Trading Program (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Sep. 30, 2013
Natural Gas Futures
Derivative
Notional amount	0		0		0
Fair value	$ 0		$ 0		$ 0
Fuel Oil and Crude Oil Futures
Derivative
Notional amount	0		0		0
Fair value	0		0		0
Natural Gas Swap
Derivative
Notional amount	117,575,000		117,575,000		152,922,500
Fair value	(91)		(91)		(169)
Fuel Oil and Crude Oil Swap
Derivative
Notional amount	0		0		1,205,000
Fair value	1		1		3
Natural Gas Option
Derivative
Notional amount	0		0		0
Fair value	0		0		0
Fuel Oil and Crude Oil Option
Derivative
Notional amount	0		0		0
Fair value	0		0		0
Natural Gas Total Contracts
Derivative
Notional amount	117,575,000		117,575,000		152,922,500
Fair value	(91)		(91)		(169)
Decrease (increase) in fuel expense	(5)	(13)	(23)	(60)
Decrease (increase) in purchased power expense	(1)	(8)	(9)	(40)
Fuel Oil and Crude Oil Total Contracts
Derivative
Notional amount	0		0		1,205,000
Fair value	1		1		3
Decrease (increase) in fuel expense	0	0	2	2
Coal Contract Total Contracts
Derivative
Fair value	0		0		0
Decrease (increase) in fuel expense	0	0	0	(1)
Closed derivative contracts
Derivative
Fair value	$ (3)		$ (3)		$ (8)