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Goodwill and intangibles - Additional Information (Detail) (USD $)
 3 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Trademarks and trade names [Member]
Jan. 31, 2003
Customer Relationship [Member]
M
Dec. 31, 2012
Customer Relationship [Member]
Dec. 31, 2012
Below market lease [Member]
M
Jan. 31, 2008
Customer relationships [Member]
M
Dec. 31, 2012
Customer relationships [Member]
Goodwill [Line Items]                      
Discount rate     11.20%                
Terminal growth rate     3.00%                
Carrying value of intangibles   $ 35,589,000 $ 31,700,000 $ 35,589,000   $ 31,700,000   $ 4,900,000      
Amortization period             80   60 180  
Impairment of Trademarks and trade names           0          
Amortization expenses 3,200,000 1,100,000 3,889,000 5,390,000 4,324,000            
Customer relationship intangible asset                     $ 630,000