Stock-based compensation - Summary of Weighted-average Assumptions (Details) - Service-based Stock Options |
3 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2020 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Expected term (in years) | 6 years 3 months 18 days | 6 years 6 months | 6 years 3 months 18 days | 6 years 2 months |
| Expected volatility | 35.13% | 35.57% | 32.02% | 32.42% |
| Risk-free interest rate | 2.55% | 2.07% | 2.68% | 2.14% |
| Expected dividend yield | 0.00% | 0.00% | 0.00% | 0.00% |