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Earnings per share - Additional Information (Detail)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Options to purchase common stock [Member]
Jun. 30, 2012
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Weighted average number diluted shares outstanding adjustment 129,524    
Outstanding options not included in diluted earnings per share   996,478 768,269