Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Derivative assets
Derivative assets:
Derivative assets	$ 697	$ 1,807	$ 7,077
Gross amounts offset in the balance sheet	(19)	(450)	0
Total derivative assets — net (a)	678	1,357	7,077
Derivative assets | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	450	1,665	6,926
Derivative assets | Derivatives not subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative assets	247	142	151
Derivative liabilities
Derivative liabilities:
Derivative liabilities	(2,263)	(1,520)	(295)
Gross amounts offset in the balance sheet	19	450	0
Total derivative liabilities — net (a)	(2,244)	(1,070)	(295)
Derivative liabilities | Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative liabilities:
Derivative liabilities	$ (2,263)	$ (1,520)	$ (295)