Derivative Activities - Schedule of Realized Gains from Cash Flow Hedges and Other Hedges Reclassified from Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue, Tax Effects			$ (1,332)		$ (4,122)
Realized Gain (Loss) Reclassified from OCI into Revenue, Net of Taxes	[1]	$ 0	2,172	$ 0	6,458
Swaps
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue, Before Taxes			1,255		3,144
Collars
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue, Before Taxes			$ 2,249		$ 7,436

[1] Amounts are net of income tax benefit of $1,332 for the three months ended September 30, 2014 and $4,122 for the nine months ended September 30, 2014. As of March 31, 2013, we elected to discontinue hedge accounting prospectively and as of December 31, 2014, all remaining accumulated other comprehensive income (“AOCI”) hedging gains had been transferred to earnings.