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Fair Value of Derivative Financial Instruments (Detail) - USD ($)
$ in Millions
Jun. 30, 2016
Dec. 31, 2015
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative assets $ 0.8  
Fair value of derivative liabilities   $ 0.7
Prepaid Expenses, Supplies, and Other Current Assets | Metal    
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative assets 0.3  
Prepaid Expenses, Supplies, and Other Current Assets | Natural Gas    
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative assets 0.3  
Other Noncurrent Assets | Natural Gas    
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative assets $ 0.2  
Accrued Liabilities | Metal    
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative liabilities   0.1
Accrued Liabilities | Natural Gas    
Derivative Instruments And Hedging Activities Disclosures [Line Items]    
Fair value of derivative liabilities   $ 0.6