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Earnings per share - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Weighted average number diluted shares outstanding adjustment 176,482      
Options to purchase common stock [Member]
       
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Outstanding options not included in diluted earnings per share 1,647,878 636,900 1,186,259 688,636