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Share-based compensation - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Common stock issued under Employee Stock Purchase Plan (in shares) 332,985 169,110 814,022 684,428