Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands	Jun. 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	585	253
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	2,761	2,731
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,520	$ 193
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (30,963)	$ (13,448)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	6,080	5,927
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	5,600	3,917
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 548	$ 1,399
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (612)	$ (326)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | bbl	2,761	2,731
Oil/Natural Gas Derivative Liabilities, Notional Amount | bbl	585	253
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 30,952	$ 13,709
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,518)	$ (187)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount | MMBTU	5,697	3,917
Oil/Natural Gas Derivative Liabilities, Notional Amount | MMBTU	5,983	5,927
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 623	$ 340
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (538)	$ (1,366)