Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Derivative assets
Derivative assets:
Derivative assets	$ 2,097	$ 1,807	$ 1,163
Gross amounts offset in the balance sheet	(101)	(450)	(159)
Total derivative assets — net (a)	1,996	1,357	1,004
Derivative assets | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	1,963	1,665	1,163
Derivative assets | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	134	142	0
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(101)	(1,520)	(1,225)
Gross amounts offset in the balance sheet	101	450	159
Total derivative liabilities — net (a)	0	(1,070)	(1,066)
Derivative liabilities | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	(101)	(1,520)	(1,204)
Derivative liabilities | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ 0	$ 0	$ (21)