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