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Fair Value Measurements (Recurring Fair Value Measures Of Assets And Liabilities) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2011
Sep. 30, 2011
Dec. 31, 2010
Sep. 30, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash Equivalents $ 224,262 $ 80,428 $ 79,622 $ 397,171
Hedging Collateral Deposits 25,118 19,701    
Total Assets 337,710 152,816    
Total Liabilities 48,210 9,674    
Total Net Assets/(Liabilities) 289,500 143,142    
Commodity Futures Contracts - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 3,516 3,292    
Over The Counter Swaps - Oil [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments   972    
Derivative Financial Instruments 54,773 6,382    
Over The Counter Swaps - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 106,115 75,113    
Derivative Financial Instruments (10,079)      
Balanced Equity Mutual Fund [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 21,696 19,882    
Common Stock - Financial Services Industry [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 4,197 4,478    
Other Common Stock [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 272 226    
Money Market Mutual Funds [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash Equivalents - Money Market Mutual Funds 180,312 32,444    
Fair Value, Inputs, Level 1 [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Hedging Collateral Deposits 25,118 19,701    
Total Assets 231,595 76,731    
Total Liabilities 3,516 3,292    
Total Net Assets/(Liabilities) 228,079 73,439    
Fair Value, Inputs, Level 1 [Member] | Commodity Futures Contracts - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 3,516 3,292    
Fair Value, Inputs, Level 1 [Member] | Over The Counter Swaps - Oil [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments   0    
Derivative Financial Instruments 0 0    
Fair Value, Inputs, Level 1 [Member] | Over The Counter Swaps - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 0 0    
Derivative Financial Instruments 0      
Fair Value, Inputs, Level 1 [Member] | Balanced Equity Mutual Fund [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 21,696 19,882    
Fair Value, Inputs, Level 1 [Member] | Common Stock - Financial Services Industry [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 4,197 4,478    
Fair Value, Inputs, Level 1 [Member] | Other Common Stock [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 272 226    
Fair Value, Inputs, Level 1 [Member] | Money Market Mutual Funds [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash Equivalents - Money Market Mutual Funds 180,312 32,444    
Fair Value, Inputs, Level 2 [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Hedging Collateral Deposits 0 0    
Total Assets 106,115 75,113    
Total Liabilities (10,079) 0    
Total Net Assets/(Liabilities) 116,194 75,113    
Fair Value, Inputs, Level 2 [Member] | Commodity Futures Contracts - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 0 0    
Fair Value, Inputs, Level 2 [Member] | Over The Counter Swaps - Oil [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments   0    
Derivative Financial Instruments 0 0    
Fair Value, Inputs, Level 2 [Member] | Over The Counter Swaps - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 106,115 75,113    
Derivative Financial Instruments (10,079)      
Fair Value, Inputs, Level 2 [Member] | Balanced Equity Mutual Fund [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 2 [Member] | Common Stock - Financial Services Industry [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 2 [Member] | Other Common Stock [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 2 [Member] | Money Market Mutual Funds [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash Equivalents - Money Market Mutual Funds 0 0    
Fair Value, Inputs, Level 3 [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Hedging Collateral Deposits 0 0    
Total Assets 0 972    
Total Liabilities 54,773 6,382    
Total Net Assets/(Liabilities) (54,773) (5,410)    
Fair Value, Inputs, Level 3 [Member] | Commodity Futures Contracts - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 0 0    
Fair Value, Inputs, Level 3 [Member] | Over The Counter Swaps - Oil [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments   972    
Derivative Financial Instruments 54,773 6,382    
Fair Value, Inputs, Level 3 [Member] | Over The Counter Swaps - Gas [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Derivative Financial Instruments 0 0    
Derivative Financial Instruments 0      
Fair Value, Inputs, Level 3 [Member] | Balanced Equity Mutual Fund [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 3 [Member] | Common Stock - Financial Services Industry [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 3 [Member] | Other Common Stock [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Other Investments 0 0    
Fair Value, Inputs, Level 3 [Member] | Money Market Mutual Funds [Member]
       
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash Equivalents - Money Market Mutual Funds $ 0 $ 0