Derivative Instruments and Hedging Activities - Components of Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Derivative assets not designated as hedging instruments:
Total derivative assets — gross	$ 41,187	$ 40,714	$ 34,625
Gross amounts offset in the balance sheet	(633)	(920)	(151)
Cash collateral received	(8,176)	(7,991)	0
Total derivative assets — net	32,378	31,803	34,474
Derivative liabilities not designated as hedging instruments:
Total derivative liabilities — gross	(633)	(920)	(151)
Gross amounts offset in the balance sheet	633	920	151
Total derivative liabilities — net	0	0	0
Not Designated as Hedging Instrument | Propane contracts
Derivative assets not designated as hedging instruments:
Total derivative assets — gross	41,187	40,714	34,625
Derivative liabilities not designated as hedging instruments:
Total derivative liabilities — gross	$ (633)	$ (920)	$ (151)