Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Derivative Asset, Current	$ 6,140	$ 0
Derivative Asset, Noncurrent	2,520	0
Liabilities:
Commodity derivative liabilities – current	13,634	12,932
Derivative Liability, Noncurrent	4,923	14,437
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Derivative Asset, Current	6,140
Derivative Asset, Noncurrent	2,520
Liabilities:
Commodity derivative liabilities – current	13,634	12,932
Derivative Liability, Noncurrent	4,923	14,437
Fair Value, Measurements, Recurring | Commodity contracts | Level 1
Assets:
Derivative Asset, Current	0
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, Current	6,140
Derivative Asset, Noncurrent	2,520
Liabilities:
Commodity derivative liabilities – current	13,634	12,932
Derivative Liability, Noncurrent	4,923	14,437
Fair Value, Measurements, Recurring | Commodity contracts | Level 3
Assets:
Derivative Asset, Current	0
Derivative Asset, Noncurrent	0
Liabilities:
Commodity derivative liabilities – current	0	0
Derivative Liability, Noncurrent	$ 0	$ 0