Derivatives, Hedges, Financial Instruments and Carbon Credits - Additional Information (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2014 Unit MMBTU	Dec. 31, 2013 Unit MMBTU	Feb. 28, 2011
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Contractual weighted-average cost	4.21	3.98
Estimated weighted-average market value	4.22
Contractual weighted-average pay rate	3.23%
Estimated market weighted-average receive rate	0.47%
Carbon credit fair value per unit	2.50	1.00
Derivative future or forward contracts in volume	13,041,000	1,530,000
The month future or forward natural gas contract period ends	2016-06	2014-10
LIBOR rate			3.23%
Interest rate swap declining balance beginning balance			$ 23,800,000
Interest rate swap declining balance ending balance			18,800,000
Interest rate contracts fixed rate description	Fixed three-month LIBOR rate of 3.23% on a declining balance
Maturity period interest rate contracts held	March 2016
Number of carbon credits	669,000	1,284,000
Assets or liabilities measured at fair value on a recurring basis transferred between Level 1 and Level 2 classifications		31,000
Senior Secured Notes [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Excess of estimated fair value over carrying value	$ 30,000,000	$ 20,000,000
Senior Secured Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notes lower range of ask or bid price	$ 106.5	$ 104.5
Notes higher range of ask or bid price	$ 107.5	$ 104.9