Derivative Activities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) t_per_M MMBTU Counterparty Institution bbl
Derivative [Line Items]
Number of financial institutions with whom we conduct derivative activities | Institution	19
Number of secured lenders not in banking credit facility | Counterparty	5
Derivatives Excluding Basis Swaps and Freight Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 13,600,000
Commodity | Basis Swaps | Natural Gas
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (7,800,000)
Volume Hedged | MMBTU	120,892,500
Commodity | Basis Swaps | Propane
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (1,200,000)
Commodity | Swaps | Freight | Level 2
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 276,000
Volume Hedged | t_per_M	5,000
2018 Commodity Contract | Basis Swaps | Propane
Derivative [Line Items]
Volume Hedged | bbl	1,362,000