Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Natural Gas	Sep. 30, 2017 MMBTU / d $ / MMBTU
2017 Commodity Contract | Puts | Purchased
Derivative [Line Items]
Weighted average deferred premium - purchases	(0.32)
Fixed price swaps | MMBTU / d	185,870	[1]
Weighted average price of hedge	3.50	[1],[2]
2017 Commodity Contract | Calls | Sold
Derivative [Line Items]
Weighted average deferred premium - sales	0.31
Fixed price swaps | MMBTU / d	17,935
Weighted average price of hedge	3.75	[3]
2017 Commodity Contract | Swaps
Derivative [Line Items]
Fixed price swaps | MMBTU / d	878,370	[1]
Weighted average price of hedge	3.21	[1]
April-December 2018 Commodity Contract | Swaps | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Fixed price swaps | MMBTU / d	160,000
Weighted average price of hedge	3.02
April-December 2018 Commodity Contract | Swaps | Counterparty Can Elect to Extend Contract
Derivative [Line Items]
Fixed price swaps | MMBTU / d	160,000
Weighted average price of hedge	3.07
2019 Commodity Contract | Swaps | Counterparty Can Elect to Double The Volume
Derivative [Line Items]
Fixed price swaps | MMBTU / d	60,000
Weighted average price of hedge	3.00

[1] Includes derivative instruments assumed in connection with the MRD Merger.

[2] Weighted average deferred premium is ($0.32).

[3] Weighted average deferred premium is $0.31.