Fair Value of Derivative Instruments on Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative liabilities, noncurrent	$ 13,900	$ 14,437
Derivative Asset	0	0
Derivative liabilities, current	27,777	12,932
Derivative liabilities	41,677	27,369
Commodity contracts
Derivatives, Fair Value [Line Items]
Derivative liabilities	41,700
Commodity contracts \| Derivative assets/liabilities - current
Derivatives, Fair Value [Line Items]
Derivative assets, current	0	0
Derivative liabilities, current	27,777	12,932
Commodity contracts \| Derivative assets/liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Derivative assets, noncurrent	0	0
Derivative liabilities, noncurrent	$ 13,900	$ 14,437

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