Share-based compensation - Additional Information (Detail)
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3 Months Ended | 9 Months Ended | ||
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Sep. 30, 2012
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Sep. 30, 2011
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Sep. 30, 2012
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Sep. 30, 2011
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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 |