Derivative Instruments and Hedging Activities - Derivative Assets and Liabilities Including Offsetting Amounts (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Assets
Derivative assets:
Fair value of derivative assets, gross	$ 3,184	$ 1,807
Gross amounts offset in the balance sheet	(146)	(450)
Total derivative assets - net	3,038	1,357
Derivative Instruments, Assets | Interest rate contracts
Derivative assets:
Fair value of derivative assets, gross	30	0
Derivative Instruments, Assets | Commodity Contract Subject to PGC and DS Mechanisms
Derivative assets:
Fair value of derivative assets, gross	3,002	1,665
Derivative Instruments, Assets | Commodity Contract Not Subject to PGC and DS Mechanisms
Derivative assets:
Fair value of derivative assets, gross	152	142
Derivative Instruments, Liabilities
Derivative liabilities:
Fair value of derivative liabilities, gross	(146)	(1,520)
Gross amounts offset in the balance sheet	146	450
Total derivative liabilities - net	0	(1,070)
Derivative Instruments, Liabilities | Commodity Contract Subject to PGC and DS Mechanisms
Derivative liabilities:
Fair value of derivative liabilities, gross	$ (146)	$ (1,520)