Natural Gas Derivative Instruments - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) MMBTU $ / MMBTU	Sep. 30, 2014 $ / MMBTU
Derivative [Line Items]
Loss on derivative instruments | $	$ (11,730)
Natural gas derivative contract
Derivative [Line Items]
Estimated cash flows, discount rate	5.50%
Last derivative contract expiration month and year	2018-12
Contracts volume, derivative instruments	7,560,000
Average price | $ / MMBTU	2.77	4.06
Natural gas derivative contract | 2015 through December 31, 2018
Derivative [Line Items]
Contracts volume, derivative instruments	9,000,000
Average price | $ / MMBTU	4.53
Cost of Sales
Derivative [Line Items]
Loss on derivative instruments | $	$ (14,259)