Derivative Instruments and Hedging Activities - Derivative Assets and Liabilities Including Offsetting Amounts (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Assets
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative assets, gross	$ 1,307	$ 1,697
Gross amounts offset in the balance sheet	(373)	(669)
Total derivative assets - net	934	1,028
Derivative Instruments, Assets | Derivatives Subject to PGC and DS Mechanisms | Commodity Contracts
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative assets, gross	1,307	1,697
Derivative Instruments, Liabilities
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative liabilities, gross	(12,964)	(2,301)
Gross amounts offset in the balance sheet	373	669
Total derivative liabilities - net	(12,591)	(1,632)
Derivative Instruments, Liabilities | Derivatives Subject to PGC and DS Mechanisms | Commodity Contracts
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative liabilities, gross	(5,584)	(2,210)
Derivative Instruments, Liabilities | Designated as Hedging Instrument | Interest Rate Contracts
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative liabilities, gross	(7,016)	0
Derivative Instruments, Liabilities | Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract]
Fair value of derivative liabilities, gross	$ (364)	$ (91)