Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Swaps - Natural Gas	Dec. 31, 2017 MMBTU / d $ / shares $ / MMBTU
April-December 2018 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged	147,500
Weighted average price of hedge | $ / shares	2.89
April-December 2018 Commodity Contract | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Volume Hedged	160,000
Weighted average price of hedge | $ / shares	3.07
2019 Commodity Contract
Derivative [Line Items]
Volume Hedged	12,329
Weighted average price of hedge | $ / MMBTU	3.01
2019 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged	85,000
Weighted average price of hedge | $ / shares	2.97