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Financial Instruments Balance Sheet Location (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Sep. 30, 2017
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 646 $ 3,239
Derivative Asset, Contract Netting 0 0
Derivative Asset, Net of Contract Netting 646 3,239
Derivative, Collateral, Right to Reclaim Cash 0 0
Derivative Asset 646 3,239
Derivative Liability, Fair Value, Gross Liability (117,175) (112,398)
Derivative Liability, Contract Netting 0 0
Derivative Liability, Net of Contract Netting (117,175) (112,398)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset 0 0
Derivative Liability (117,175) (112,398)
Designated As Hedging Instrument [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Derivative Liability, Fair Value, Gross Liability (114,175) (112,076)
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 Credits and Other Liabilities [Member] | Interest Rate Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (114,175) (112,076)
Nondesignated [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 646 3,239
Derivative Liability, Fair Value, Gross Liability (3,000) (322)
Nondesignated [Member] | Other Current Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 456 2,436
Nondesignated [Member] | Deferred Charges and Other Assets [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 190 803
Nondesignated [Member] | Other Current Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (2,738) (322)
Nondesignated [Member] | Deferred Credits and Other Liabilities [Member] | Commodity Contract [Member]    
Derivatives Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability $ (262) $ 0