Income taxes - Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax (benefit) expense	$ 1,242	$ (273)	$ 4,710	$ (500)
Effective tax rate	27.30%	0.40%	(4.40%)	0.50%