Derivative Instruments - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) Entity	Dec. 31, 2014 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative assets	$ 112,360	$ 164,878
Commodity contracts
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative assets	$ 112,400
Number of derivative counterparties | Entity	7
Commodity contracts | Credit Concentration Risk
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Number of derivative counterparties | Entity	4
Commodity contracts | Natural Gas
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Third-party quoted forward prices	NYMEX Henry Hub gas
Commodity contracts | Crude Oil
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Third-party quoted forward prices	West Texas Intermediate crude oil