Derivative Instruments and Hedging Activities - Derivative Assets and Liabilities Including Offsetting Amounts (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Assets
Derivative assets:
Fair value of derivative assets, gross	$ 1,807	$ 4,510
Gross amounts offset in the balance sheet	(450)	(247)
Total derivative assets - net	1,357	4,263
Derivative Instruments, Assets | Commodity Contract Subject to PGC and DS Mechanisms
Derivative assets:
Fair value of derivative assets, gross	1,665	4,472
Derivative Instruments, Assets | Commodity Contract Not Subject to PGC and DS Mechanisms
Derivative assets:
Fair value of derivative assets, gross	142	38
Derivative Instruments, Liabilities
Derivative liabilities:
Fair value of derivative liabilities, gross	(1,520)	(557)
Gross amounts offset in the balance sheet	450	247
Total derivative liabilities - net	(1,070)	(310)
Derivative Instruments, Liabilities | Commodity Contract Subject to PGC and DS Mechanisms
Derivative liabilities:
Fair value of derivative liabilities, gross	(1,520)	(499)
Derivative Instruments, Liabilities | Commodity Contract Not Subject to PGC and DS Mechanisms
Derivative liabilities:
Fair value of derivative liabilities, gross	$ 0	$ (58)