Net income per share attributable to common stockholders - Additional Information (Details)	1 Months Ended
	Jun. 30, 2014 vote
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stockholders' Equity Note, 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, Voting Rights, Number	1
Common Class B [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common Stock, Voting Rights, Number	10