Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands	Sep. 30, 2018 USD ($) MMBTU bbl	Dec. 31, 2017 USD ($) MMBTU bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	622	253
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,889	2,731
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 917	$ 193
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (34,941)	$ (13,448)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	7,939	5,927
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	8,382	3,917
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 427	$ 1,399
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (598)	$ (326)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,889	2,731
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	622	253
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 35,083	$ 13,709
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (917)	$ (187)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	9,588	3,917
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	6,734	5,927
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 682	$ 340
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (392)	$ (1,366)