Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Swaps - Natural Gas	Dec. 31, 2018 MMBTU / d $ / MMBTU
2019 Commodity Contract
Derivative [Line Items]
Volume Hedged | MMBTU / d	1,318,041
Weighted average price of hedge | $ / MMBTU	2.86
2019 Commodity Contract | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Volume Hedged | MMBTU / d	150,000
Weighted average price of hedge | $ / MMBTU	2.81
2020 Commodity Contract | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Volume Hedged | MMBTU / d	80,000
Weighted average price of hedge | $ / MMBTU	2.77