Fair Value Measurements (Narrative) (Details) (USD $)	9 Months Ended	12 Months Ended
	Jun. 30, 2011	Sep. 30, 2010
Hedging collateral deposits	$ 37,984,000	$ 11,134,000
NYMEX Futures [Member] | Fair Value, Inputs, Level 1 [Member]
Hedging collateral deposits	5,600,000	10,100,000
Crude Oil Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Hedging collateral deposits	32,400,000	1,000,000
Price Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Reduction in fair market value of price swap agreements counterparty credit risk	400,000	1,000,000
Price Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Inputs, Level 3 [Member]
Reduction in fair market value of price swap agreements counterparty credit risk	100,000	300,000
Exchange-Traded Futures Contracts [Member]
Hedging collateral deposits	5,600,000	10,100,000
Over-The-Counter Crude Oil Swap Agreements [Member]
Hedging collateral deposits	32,400,000	1,000,000
Fair Value, Inputs, Level 1 [Member]
Hedging collateral deposits	$ 37,984,000	$ 11,134,000