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Line of Credit - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Line of Credit Facility [Line Items]      
Borrowings under credit facility $ 125.0 $ 131.5  
Repayments of credit facility 140.0 133.0  
Available credit facility 45.0    
Line of credit 80.0    
Outstanding letters of credit 25.0    
Interest rate 4.50%   4.50%
Standby Letter of Credit
     
Line of Credit Facility [Line Items]      
Reserved under standby letter of credit $ 10.0