Fair Value of Derivative Instruments on Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 96,211	$ 128,981
Derivative assets, noncurrent	16,149	35,897
Derivative assets	112,360	164,878
Derivative liabilities, current	0	0
Derivative liabilities	0	0
Commodity contracts
Derivatives, Fair Value [Line Items]
Derivative assets	112,400
Commodity contracts | Derivative assets/liabilities - current
Derivatives, Fair Value [Line Items]
Derivative assets, current	96,211	128,981
Derivative liabilities, current	0	0
Commodity contracts | Derivative assets/liabilities - noncurrent
Derivatives, Fair Value [Line Items]
Derivative assets, noncurrent	16,149	35,897
Derivative liabilities, noncurrent	$ 0	$ 0