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Financial Assets and Liabilities Accounted For at Fair Value on Recurring Basis (Detail) - USD ($)
Jun. 30, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and Capital Lease Obligations $ 336,038 $ 332,527
Convertible senior notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and Capital Lease Obligations 136,692 134,601
Term loan credit agreement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and Capital Lease Obligations 191,439 189,027
Industrial revenue bond    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and Capital Lease Obligations 1,399 1,645
Capital lease obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt and Capital Lease Obligations 6,508 7,254
Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 1 | Convertible senior notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 1 | Term loan credit agreement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 1 | Industrial revenue bond    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 1 | Capital lease obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 379,369 381,335
Fair Value, Inputs, Level 2 | Convertible senior notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 186,525 188,490
Fair Value, Inputs, Level 2 | Term loan credit agreement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 192,844 192,845
Fair Value, Inputs, Level 2 | Industrial revenue bond    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 2 | Capital lease obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 7,907 8,899
Fair Value, Inputs, Level 3 | Convertible senior notes    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 3 | Term loan credit agreement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 3 | Industrial revenue bond    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure 1,399 1,645
Fair Value, Inputs, Level 3 | Capital lease obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, Fair Value Disclosure $ 6,508 $ 7,254