Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Derivative assets
Derivative assets:
Derivative assets	$ 1,051	$ 1,807	$ 2,157
Gross amounts offset in the balance sheet	(37)	(450)	(133)
Total derivative assets — net (a)	1,014	1,357	2,024
Derivative assets | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	860	1,665	2,104
Derivative assets | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	191	142	53
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(589)	(1,520)	(152)
Gross amounts offset in the balance sheet	37	450	133
Total derivative liabilities — net (a)	(552)	(1,070)	(19)
Derivative liabilities | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	(589)	(1,520)	(150)
Derivative liabilities | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ 0	$ 0	$ (2)