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Compensation Expense Associated with Stock Settled Performance Units (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Performance Unit Awards 2012
     
Unrecognized Share Based Compensation Expense [Line Items]      
Stock-based compensation expense associated with Stock-Settled Performance Units $ 766    
Performance Unit Awards 2011
     
Unrecognized Share Based Compensation Expense [Line Items]      
Stock-based compensation expense associated with Stock-Settled Performance Units 1,856 1,392  
Performance Unit Awards 2010
     
Unrecognized Share Based Compensation Expense [Line Items]      
Stock-based compensation expense associated with Stock-Settled Performance Units $ 1,039 $ 1,039 $ 779