Stockholders' equity - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 157,392,000	$ 179,028,000	$ 180,407,000	$ 160,206,000
Other comprehensive loss, net of taxes of zero	(243,000)	5,317,000	(555,000)	3,808,000
Ending balance	157,141,000	181,518,000	157,141,000	181,518,000
Other comprehensive loss, tax			0
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			1,951,000
Other comprehensive loss, net of taxes of zero			(555,000)
Ending balance	1,396,000		1,396,000
Defined Benefit Pension Plan Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(216,000)
Other comprehensive loss, net of taxes of zero			0
Ending balance	(216,000)		(216,000)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1,423,000	(9,380,000)	1,735,000	(7,871,000)
Ending balance	$ 1,180,000	$ (4,063,000)	$ 1,180,000	$ (4,063,000)