Derivative Activities - Additional Information (Detail)	Sep. 30, 2017 USD ($) t_per_M MMBTU bbl
Derivatives Excluding Basis Swaps and Freight Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (14,600,000)
Commodity | Basis Swaps | Freight
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 45,000
Volume Hedged | t_per_M	5,000
Commodity | Basis Swaps | Natural Gas
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (4,700,000)
Volume Hedged | MMBTU	130,120,000
Commodity | Basis Swaps | Propane
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 1,100,000
2017 Commodity Contract | Basis Swaps | Propane
Derivative [Line Items]
Volume Hedged | bbl	659,000
2018 Commodity Contract | Basis Swaps | Propane
Derivative [Line Items]
Volume Hedged | bbl	750,000