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Stock-based compensation - Summary of Weighted-average Assumptions (Details) - Service-based Stock Options
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 6 years 3 months 18 days 6 years 2 months 12 days 5 years 10 months 24 days
Expected volatility 32.02% 32.42% 36.50%
Risk-free interest rate 2.68% 2.14% 1.34%
Expected dividend yield 0.00% 0.00% 0.00%