Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Natural Gas	Jun. 30, 2018 MMBTU / d $ / shares $ / MMBTU
July-December 2018 Commodity Contract | Swaps | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Volume Hedged	160,000
Weighted average price of hedge | $ / shares	3.07
2019 Commodity Contract | Swaps
Derivative [Line Items]
Volume Hedged	514,589
Weighted average price of hedge | $ / MMBTU	2.81
2019 Commodity Contract | Swaps | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Volume Hedged	130,000
Weighted average price of hedge | $ / shares	2.81
2019 Commodity Contract | Swaps | Counterparty Can Elect To Double Volume
Derivative [Line Items]
Volume Hedged	220,000
Weighted average price of hedge | $ / shares	2.89
2020 Commodity Contract | Swaps | Counterparty Can Elect To Double Volume
Derivative [Line Items]
Volume Hedged	10,000
Weighted average price of hedge | $ / shares	2.75
October-December 2018 Commodity Contract | Calls
Derivative [Line Items]
Volume Hedged	70,000
Weighted average price of hedge | $ / MMBTU	3.10	[1]
Weighted average deferred premium | $ / MMBTU	0.16

[1] Weighted average deferred premium of $0.16.