Fair Value Measurements (Narrative) (Details) (USD $)	3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	$ 25,118,000	$ 19,701,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	25,118,000	19,701,000
Fair Value, Inputs, Level 1 [Member] | NYMEX Futures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	4,500,000	5,500,000
Fair Value, Inputs, Level 1 [Member] | Price Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	20,600,000	14,200,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	0	0
Fair Value, Inputs, Level 2 [Member] | Price Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reduction in fair market value of price swap agreements based on assessment of counterparty credit risk	2,200,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedging collateral deposits	0	0
Fair Value Inputs Level 2 And Level 3 [Member] | Price Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Reduction in fair market value of price swap agreements based on assessment of counterparty credit risk	$ 100,000	$ 2,000,000