Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Derivative assets
Derivative assets:
Derivative assets	$ 1,336	$ 3,184	$ 697
Gross amounts offset in the balance sheet	(266)	(146)	(19)
Total derivative assets — net (a)	1,070	3,038	678
Derivative assets | Designated as Hedging Instrument | Interest rate contracts
Derivative assets:
Derivative assets	0	30	0
Derivative assets | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	1,336	3,002	450
Derivative assets | Not designated as hedging instruments | Commodity contracts
Derivative assets:
Derivative assets	0	152	247
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(2,388)	(146)	(2,263)
Gross amounts offset in the balance sheet	266	146	19
Total derivative liabilities — net (a)	(2,122)	0	(2,244)
Derivative liabilities | Designated as Hedging Instrument | Interest rate contracts
Derivative liabilities:
Derivative liabilities	(1,655)	0	0
Derivative liabilities | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	(499)	(146)	(2,263)
Derivative liabilities | Not designated as hedging instruments | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ (234)	$ 0	$ 0