Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Commodity derivative liabilities – current	$ 80,641	$ 27,777
Derivative Liability, Noncurrent	37,570	13,900
Fair Value, Measurements, Recurring | Commodity contracts
Liabilities:
Commodity derivative liabilities – current	80,641	27,777
Derivative Liability, Noncurrent	37,570	13,900
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Liabilities:
Commodity derivative liabilities – current	0	0
Derivative Liability, Noncurrent	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Liabilities:
Commodity derivative liabilities – current	80,641	27,777
Derivative Liability, Noncurrent	37,570	13,900
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Liabilities:
Commodity derivative liabilities – current	0	0
Derivative Liability, Noncurrent	$ 0	$ 0