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Derivatives, Hedges, Financial Instruments and Carbon Credits - Additional Information (Detail)
6 Months Ended 12 Months Ended
Jun. 30, 2015
USD ($)
MMBTU
Unit
lb
oz
$ / shares
$ / Transaction
$ / MMBTU
$ / lb
$ / €
Dec. 31, 2014
USD ($)
MMBTU
Unit
lb
oz
$ / shares
$ / Transaction
$ / MMBTU
$ / lb
$ / oz
$ / €
Jun. 30, 2015
EUR (€)
MMBTU
Unit
lb
oz
$ / Transaction
$ / €
Dec. 31, 2014
EUR (€)
MMBTU
Unit
lb
oz
$ / Transaction
$ / €
Feb. 28, 2011
USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]          
Contractual weighted-average cost | $ / MMBTU 3.09 3.24      
Estimated weighted-average market value | $ / MMBTU 2.92        
Contractual weighted-average exchange rate | $ / € 1.14 1.27 1.14 1.27  
Contractual weighted-average pay rate 3.23%        
Estimated market weighted-average receive rate 0.41%        
Carbon credit fair value per unit | $ / Transaction 2.35 2.50 2.35 2.50  
Derivative future or forward contracts in pounds | lb 1,950,000 1,750,000 1,950,000 1,750,000  
Derivative future or forward contracts in volume | MMBTU 4,655,000 8,279,000 4,655,000 8,279,000  
Future or forward copper contracts period 2015-12 2015-05      
Weighted-average cost per pound of future or forward copper contracts | $ / lb 2.77 2.98      
The month future or forward natural gas contract period ends 2016-06 2016-06      
Future or forward contract in ounces | oz 0 3,000 0 3,000  
Future or forward platinum contracts settlement period   2015-04      
Weighted-average cost per ounce of future or forward platinum contracts | $ / oz   1,224.26      
Amount of foreign currency contract | €     € 280,000 € 819,000  
Future or forward foreign exchange contracts settlement period 2015-09 2015-05      
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          
Number of carbon credits | Unit 1,727,000 1,112,000 1,727,000 1,112,000  
Assets or liabilities measured at fair value on a recurring basis transferred between Level 1 and Level 2 classifications $ 0        
Senior Secured Notes [Member]          
Derivative Instruments and Hedging Activities Disclosures [Line Items]          
Excess of estimated fair value over carrying value $ 30,000,000 $ 17,000,000      
Senior Secured Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]          
Derivative Instruments and Hedging Activities Disclosures [Line Items]          
Notes based on quoted price | $ / shares $ 107.0 $ 104.0      
Foreign Exchange Contracts [Member]          
Derivative Instruments and Hedging Activities Disclosures [Line Items]          
Contractual weighted-average exchange rate | $ / € 1.14   1.14    
Total estimated market weighted-average exchange rate | $ / € 1.11