Fair Value of Derivative Instruments on Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 36,820	$ 27,777
Derivative Asset, Noncurrent	32	0
Derivative assets	32	0
Derivative Liability, Noncurrent	16,108	13,900
Derivative Liability	52,928	41,677
Commodity contracts
Derivatives, Fair Value [Line Items]
Derivative Liability	52,900
Commodity contracts | Current Derivative Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Current	0	0
Derivative liabilities	36,820	27,777
Commodity contracts | Noncurrent Derivative Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Noncurrent	32	0
Derivative Liability, Noncurrent	$ 16,108	$ 13,900