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, 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	2,416	253
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 24,332	$ 193
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl	415	2,731
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (646)	$ (13,448)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU	5,745	5,927
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 417	$ 1,399
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU	9,314	3,917
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (1,272)	$ (326)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | bbl	415	2,731
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 646	$ 13,709
Oil/Natural Gas Derivative Liabilities Notional Amount | bbl	2,416	253
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (24,009)	$ (187)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount | MMBTU	10,236	3,917
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,373	$ 340
Oil/Natural Gas Derivative Liabilities Notional Amount | MMBTU	4,823	5,927
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (393)	$ (1,366)