Quarterly financial summary - Schedule of Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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
Gross profit	48,064	52,520	43,348	37,191	40,491	46,919	38,969	36,645	40,208	181,123	162,741	164,725
Net income	$ (341)	$ 8,002	$ 6,517	$ 3,706	$ (17,914)	$ 9,672	$ 3,915	$ 1,248	$ 690	$ 17,884	$ 15,525	$ 33,475
Net income (loss) per share:
Basic (in USD per share)	$ (0.01)	$ 0.16	$ 0.13	$ 0.08	$ (0.37)	$ 0.20	$ 0.08	$ 0.03	$ 0.01	$ 0.37	$ 0.33	$ 0.74
Diluted (in USD per share)	$ (0.01)	$ 0.16	$ 0.13	$ 0.07	$ (0.37)	$ 0.20	$ 0.08	$ 0.03	$ 0.01	$ 0.35	$ 0.32	$ 0.68