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Earnings per share - Additional Information (Detail)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Earnings Per Share [Abstract]        
Shares not included in computing diluted earnings per share as effects were anti-dilutive 177,871 107,727 177,871 122,056