Derivative Activities - Additional Information (Detail)	Sep. 30, 2018 USD ($) t_per_M MMBTU bbl
Derivatives Excluding Basis Swaps and Freight Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (104,800,000)
Commodity | Basis Swaps | Natural Gas
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (1,300,000)
Volume Hedged | MMBTU	73,660,000
Commodity | Basis Swaps | Propane
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (2,000,000)
Commodity | Swaps | Freight | Level 2
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 301,000
Volume Hedged | t_per_M	5,000
2019 Commodity Contract | Basis Swaps | Propane
Derivative [Line Items]
Volume Hedged | bbl	1,943,000