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Leases, User Charges and Commitments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 12 Months Ended
Dec. 12, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Lease and Rental Expense [Line Items]        
Rental expense   $ 8.8 $ 9.9 $ 8.0
Rail transportation charges   67.0 64.0 54.0
Number of containers to be purchased at lease expiration date 4,097      
Total price of containers to be purchased at lease expiration date 14.9      
Leases expiry date 2013      
Transportation Costs
        
Lease and Rental Expense [Line Items]        
Rental expense   $ 9.3 $ 9.4 $ 9.3