Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)	12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Derivative
Unrealized gains/losses on derivatives	$ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives	310,000,000	$ (472,000,000)
FTP transaction limit	$ 130,000,000
Maximum hedge volume	75.00%
Market value limitation of outstanding construction materials hedging transactions	$ 100,000,000
Portfolio value at risk limit for foreign currency transactions	5,000,000
Interest Rate Swap
Derivative
Amount of gain (loss) recognized in income on derivatives	(89,000,000)	(101,000,000)
Fair value	(317,000,000)	(410,000,000)
Commodity Contract Derivatives
Derivative
Fair value	60,000,000	(60,000,000)
Commodity derivatives under FTP
Derivative
Amount of gain (loss) recognized in income on derivatives	(8,000,000)	(36,000,000)
Fair value	$ 0	$ (5,000,000)
Coal Contract Derivatives
Derivative
Number of contracts	13	20
Notional amount	20,000,000	17,000,000
Fair value	$ 58,000,000	$ (67,000,000)
Natural Gas Contract Derivatives
Derivative
Number of contracts	61	53
Notional amount	359,000,000	271,000,000
Fair value	$ 2,000,000	$ 7,000,000
Natural Gas Swap
Derivative
Notional amount	0	2,800,000
Fair value	$ 0	$ (5,000,000)