Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Derivative assets
Derivative assets:
Derivative assets	$ 1,258	$ 3,184	$ 1,051
Gross amounts offset in the balance sheet	(211)	(146)	(37)
Total derivative assets — net (a)	1,047	3,038	1,014
Derivative assets | Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Derivative assets	1,258	3,002	860
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	191
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(2,779)	(146)	(589)
Gross amounts offset in the balance sheet	211	146	37
Total derivative liabilities — net (a)	(2,568)	0	(552)
Derivative liabilities | Derivatives subject to PGC and DS mechanisms:
Derivative liabilities:
Derivative liabilities	(211)	(146)	(589)
Derivative liabilities | Designated as Hedging Instrument | Interest rate contracts
Derivative liabilities:
Derivative liabilities	(2,528)	0	0
Derivative liabilities | Not designated as hedging instruments | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ (40)	$ 0	$ 0