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Long-Term Debt and Financing Arrangements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 12 Months Ended
Aug. 01, 2011
Dec. 31, 2012
Dec. 31, 2011
Mar. 31, 2011
Schedule Of Debt [Line Items]        
Maximum unsecured borrowing capacity   $ 50.0   $ 10.0
Interest rate under the Credit Agreement   1.75%    
Line of credit, expiration date   March 2014    
Minimum net worth   300.0    
Commitment fee charged on the unused line of credit   0.375%    
Outstanding letters of credit   3.7    
Letter of credit expiration, year   2013    
Unused and available borrowings under bank revolving line of credit   46.3 47.4  
Number of chassis leased 3,126      
Chassis, lease agreement period (years) 10 years      
Capital lease interest paid   $ 1.0 $ 0.3  
Maximum
        
Schedule Of Debt [Line Items]        
Cash flow leverage ratio   2.0