Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Dec. 31, 2015 Contract	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Maximum correlation between price of oil & natural gas in market and underlying price basis indicative in the derivative contract	100.00%
Accumulated other comprehensive income (loss)	$ 35,057		$ 40,700	$ 83,315	$ 11,200	$ (18,200)	$ (2,100)
Accumulated other comprehensive income, to be reclassified into earnings in the next twelve months	37,700
Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ 41,100		$ 46,500	$ 86,800	$ 13,200	$ (17,800)	$ (1,400)
Scenario, Forecast [Member] | Natural Gas (MMcfe) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of contracts no longer qualifying as cash flow hedges | Contract		3