Derivatives, Hedges, Financial Instruments and Carbon Credits - Additional Information (Detail) (USD $)	1 Months Ended			12 Months Ended
	Feb. 28, 2011	Sep. 30, 2008	Apr. 30, 2008	Dec. 31, 2012 lb	Dec. 31, 2011 lb	Dec. 31, 2010
Contractual weighted-average pay rate				3.23%
Estimated market weighted-average receive rate				0.49%
Carbon credit fair value per unit				0.50	3.15
Carbon credit lower range of ask or bid price					$ 1.00
Carbon credit higher range of ask or bid price					3.00
Derivative future or forward contracts in Pounds				625,000	375,000
Weighted-average cost per pound of future or forward copper contracts				3.53	3.42
Future or forward copper contracts period				May 2013	May 2012
Interest rate contracts fixed rate description	Fixed three-month LIBOR rate of 3.23% on a declining balance	Fixed three-month LIBOR rate of 3.595% on $25 million	Fixed three-month LIBOR rate of 3.24% on $25 million
Maturity period interest rate contracts held	March 2016	April 2012	April 2012
Interest rate contracts at cost basis		400,000
Interest rate contracts LIBOR rate		3.595%	3.24%
Notional amount of Interest rate contracts		25,000,000	25,000,000
LIBOR rate	3.23%
Interest rate swap declining balance beginning balance	23,800,000
Interest rate swap declining balance ending balance	18,800,000
Assets or liabilities measured at fair value on a recurring basis transferred between Level 1 and Level 2 classifications