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Compensation Expense Associated with Stock-Settled Performance Units (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Performance Unit Awards 2012
   
Unrecognized Share Based Compensation Expense [Line Items]    
Stock-based compensation expense associated with Stock-Settled Performance Units $ 255  
Performance Unit Awards 2011
   
Unrecognized Share Based Compensation Expense [Line Items]    
Stock-based compensation expense associated with Stock-Settled Performance Units 464 464
Performance Unit Awards 2010
   
Unrecognized Share Based Compensation Expense [Line Items]    
Stock-based compensation expense associated with Stock-Settled Performance Units $ 260 $ 260