Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Derivative Asset, Noncurrent	$ 32	$ 0
Liabilities:
Commodity derivative liabilities – current	36,820	27,777
Derivative Liability, Noncurrent	16,108	13,900
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Derivative Asset, Noncurrent	32
Liabilities:
Commodity derivative liabilities – current	36,820	27,777
Derivative Liability, Noncurrent	16,108	13,900
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Derivative Asset, Noncurrent	0
Liabilities:
Commodity derivative liabilities – current	0	0
Derivative Liability, Noncurrent	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Level 2
Assets:
Derivative Asset, Noncurrent	32
Liabilities:
Commodity derivative liabilities – current	36,820	27,777
Derivative Liability, Noncurrent	16,108	13,900
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Derivative Asset, Noncurrent	0
Liabilities:
Commodity derivative liabilities – current	0	0
Derivative Liability, Noncurrent	$ 0	$ 0