Summary of significant accounting policies - Reconciliation of Sales Allowances (Details) - Sales Allowances - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2020 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Sales Allowances [Roll Forward] | ||||
| Beginning balance | $ 7,774 | $ 6,545 | $ 8,458 | $ 11,927 |
| Charges | 6,787 | 29,576 | 26,971 | 25,680 |
| Deductions | (8,016) | (28,508) | (27,655) | (29,149) |
| Ending balance | $ 6,545 | $ 7,613 | $ 7,774 | $ 8,458 |