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Financial Instruments Balance Sheet Location (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Sep. 30, 2016
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 3,096 $ 44,141
Derivative Asset, Contract Netting 0 (39,290)
Derivative Asset, Net of Contract Netting 3,096 4,851
Derivative, Collateral, Right to Reclaim Cash 0 6,775
Derivative Asset 3,096 11,626
Derivative Liability, Fair Value, Gross Liability (117,100) (323,684)
Derivative Liability, Contract Netting 0 (39,290)
Derivative Liability, Net of Contract Netting (117,100) (284,394)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset 0 (43,575)
Derivative Liability (117,100) (240,819)
Designated As Hedging Instrument [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 8,790
Derivative Liability, Fair Value, Gross Liability (116,455) (292,171)
Designated As Hedging Instrument [Member] | Other Current Assets [Member] | Interest Rate Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Designated As Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   6,612
Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member] | Interest Rate Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   2,178
Designated As Hedging Instrument [Member] | Other Current Liabilities [Member] | Interest Rate Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (22,199) (68,481)
Designated As Hedging Instrument [Member] | Other Current Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability   (21,903)
Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member] | Interest Rate Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (94,256) (198,008)
Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability   (3,779)
Nondesignated [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 3,096 35,351
Derivative Liability, Fair Value, Gross Liability (645) (31,513)
Nondesignated [Member] | Other Current Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 3,096 21,186
Nondesignated [Member] | Deferred Charges And Other Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   14,165
Nondesignated [Member] | Other Current Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability $ (645) (18,812)
Nondesignated [Member] | Deferred Credits And Other Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability   $ (12,701)