Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Detail) bbl in Thousands, MMBTU in Thousands, $ in Thousands	Dec. 31, 2017 USD ($) MMBTU bbl	Dec. 31, 2016 USD ($) MMBTU bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | bbl	253	227
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 193	$ 206
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl	2,731	944
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (13,448)	$ (4,400)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU	5,927	0
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,399	$ 0
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU	3,917	1,299
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (326)	$ (1,357)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | bbl	2,731	944
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 13,709	$ 4,580
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl	253	227
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (187)	$ (206)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU	3,917	1,299
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 340	$ 1,393
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU	5,927	0
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,366)	$ 0