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Stockholders' equity (deficit) and redeemable convertible preferred stock - Initial public offering and Follow-on offering (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended
Jul. 01, 2014
Nov. 30, 2014
Dec. 31, 2015
Dec. 31, 2014
Class of Stock [Line Items]        
Preferred Stock, Shares Issued     0 0
IPO [Member] | Common Class A [Member]        
Class of Stock [Line Items]        
Stock Issued During Period, Shares, New Issues 8,900,000.0      
Share Price $ 24.00      
Stock Sold by Stockholders During Period, Shares, Underwritten Public Offering 11,570,000      
Stock Sold by Stockholders During Period, Shares, Over-allotment Option in Underwritten Public Offering 2,670,000      
Proceeds from Issuance Initial Public Offering $ 200.8 $ 93.2    
Underwriting Discounts And Commissions 12.8      
Total Offering Costs $ 6.2      
Follow-on Offering [Member] | Common Class A [Member]        
Class of Stock [Line Items]        
Stock Issued During Period, Shares, New Issues   1,287,533    
Share Price   $ 75.00    
Stock Sold by Stockholders During Period, Shares, Underwritten Public Offering   10,627,042    
Stock Sold by Stockholders During Period, Shares, Over-allotment Option in Underwritten Public Offering   1,554,075    
Underwriting Discounts And Commissions   $ 3.4    
Total Offering Costs   $ 1.5