Income taxes - Changes in valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in the valuation allowance
Balance at the beginning of the year	$ 41,646	$ 33,810	$ 34,990
Decrease credited to operations	(3)	(7)	(1,180)
Additions charged to operations	4,816	7,843
Balance at the end of the year	$ 46,459	41,646	$ 33,810
Period of cumulative results for determination of releasing valuation allowance	3 years
Uncertain tax positions	$ 0	$ 0