Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Derivative assets
Derivative assets:
Derivative assets	$ 944	$ 3,184	$ 2,097
Gross amounts offset in the balance sheet	(19)	(146)	(101)
Total derivative assets — net (a)	925	3,038	1,996
Derivative assets | Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Derivative assets	944	3,002	1,963
Derivative assets | Designated as Hedging Instrument | Interest rate contracts
Derivative assets:
Derivative assets	0	30	0
Derivative assets | Not designated as hedging instruments | Commodity contracts
Derivative assets:
Derivative assets	0	152	134
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(7,327)	(146)	(101)
Gross amounts offset in the balance sheet	19	146	101
Total derivative liabilities — net (a)	(7,308)	0	0
Derivative liabilities | Derivatives subject to PGC and DS mechanisms:
Derivative liabilities:
Derivative liabilities	(3,085)	(146)	(101)
Derivative liabilities | Designated as Hedging Instrument | Interest rate contracts
Derivative liabilities:
Derivative liabilities	(4,211)	0	0
Derivative liabilities | Not designated as hedging instruments | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ (31)	$ 0	$ 0