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Net income per share attributable to common stockholders - Additional Information (Details)
9 Months Ended
Sep. 30, 2015
vote
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]  
Outstanding shares less than 10% of aggregate shares outstanding, conversion ratio 1
Common Class A [Member]  
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]  
Common stock, number of voting rights 1
Common Class B [Member]  
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]  
Common stock, number of voting rights 10