Summary of significant accounting policies - Net Sales in United States and Outside of United States (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Mar. 31, 2020 |
Dec. 31, 2019 |
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2018 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Mar. 31, 2020 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Revenues from External Customers and Long-Lived Assets [Line Items] | ||||||||||||
| Net sales | $ 74,712 | $ 80,760 | $ 67,615 | $ 59,764 | $ 66,141 | $ 78,571 | $ 63,889 | $ 59,055 | $ 65,920 | $ 282,851 | $ 267,435 | $ 269,888 |
| U.S. | ||||||||||||
| Revenues from External Customers and Long-Lived Assets [Line Items] | ||||||||||||
| Net sales | 59,797 | 255,284 | 241,159 | 243,299 | ||||||||
| International | ||||||||||||
| Revenues from External Customers and Long-Lived Assets [Line Items] | ||||||||||||
| Net sales | $ 6,344 | $ 27,567 | $ 26,276 | $ 26,589 | ||||||||