Commodity Derivative Positions (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) bbl counterparty $ / bbl $ / bbl	Jan. 31, 2018 bbl $ / bbl
Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / bbl	49.12
Settlement Agreement [Member]
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Liability	$ 1,482
Fourth Quarter 2019 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	6,398
Derivative, Swap Type, Average Fixed Price | $ / bbl	50.97
Fair Value Asset	$ 0
Fair Value Liability	$ 2,528
Fourth Quarter 2020 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	2,000
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.29
Fair Value Asset	$ 0
Fair Value Liability	$ 228
Third Quarter 2019 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	6,397
Derivative, Swap Type, Average Fixed Price | $ / bbl	50.97
Fair Value Asset	$ 0
Fair Value Liability	$ 2,886
Third Quarter 2020 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	2,000
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.29
Fair Value Asset	$ 0
Fair Value Liability	$ 283
Second Quarter 2019 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	6,421
Derivative, Swap Type, Average Fixed Price | $ / bbl	50.97
Fair Value Asset	$ 0
Fair Value Liability	$ 3,336
Second Quarter 2020 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	2,000
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.29
Fair Value Asset	$ 0
Fair Value Liability	$ 353
First Quarter 2019 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	6,446
Derivative, Swap Type, Average Fixed Price | $ / bbl	50.97
Fair Value Asset	$ 0
Fair Value Liability	$ 3,845
First Quarter 2020 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	2,000
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.29
Fair Value Asset	$ 0
Fair Value Liability	$ 441
Fourth Quarter 2018 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	7,955
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.15
Fair Value Asset	$ 0
Fair Value Liability	$ 5,357
Third Quarter 2018 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	7,955
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.15
Fair Value Asset	$ 0
Fair Value Liability	$ 6,241
Second Quarter 2018 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	7,984
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.15
Fair Value Asset	$ 0
Fair Value Liability	$ 7,075
First Quarter 2018 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Derivative, Nonmonetary Notional Amount | bbl	8,013
Derivative, Swap Type, Average Fixed Price | $ / bbl	51.14
Fair Value Asset	$ 0
Fair Value Liability	$ 7,622
Fourth Quarter 2017 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Third Quarter 2017 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Second Quarter 2017 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
First Quarter 2017 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Instrument 1	Swaps
Commodity contracts
Derivative Instruments Related to Oil and Gas Production [Line Items]
Number Of Derivative Counterparty | counterparty	5
Subsequent Event [Member] | 2018 [Member] | Crude Oil
Derivative Instruments Related to Oil and Gas Production [Line Items]
Derivative, Nonmonetary Notional Amount | bbl		6,227
Derivative, Swap Type, Average Fixed Price | $ / bbl		50.70