Long-term debt - Subsequent Event (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Oct. 01, 2020	Sep. 30, 2020	Jun. 30, 2019	Sep. 30, 2020
Subsequent Event [Line Items]
Shares issued		$ 46,323	$ 56,528
Hercules
Subsequent Event [Line Items]
Shares issued				$ 2,000
Hercules | Sales Agreement | Subsequent events
Subsequent Event [Line Items]
Shares issued	$ 50,000
At the market offering
Subsequent Event [Line Items]
Shares issued (in shares)				2,830,713
Net proceeds				$ 46,300
At the market offering | Hercules | Sales Agreement | Subsequent events
Subsequent Event [Line Items]
Shares issued (in shares)	98,100
Share issue price	$ 19.05
Net proceeds	$ 1,800