Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Derivative assets
Derivative assets:
Derivative assets	$ 2,157	$ 4,510	$ 1,179
Gross amounts offset in the balance sheet	(133)	(247)	(298)
Total derivative assets — net	2,024	4,263	881
Derivative assets | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	2,104	4,472	1,179
Derivative assets | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	53	38	0
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(152)	(557)	(3,668)
Gross amounts offset in the balance sheet	133	247	298
Total derivative liabilities — net	(19)	(310)	(3,370)
Derivative liabilities | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	(150)	(499)	(3,466)
Derivative liabilities | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ (2)	$ (58)	$ (202)