Stockholders' equity - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 169,659,000	$ 167,238,000	$ 180,407,000	$ 160,206,000
Other comprehensive loss, net of taxes of zero	2,031,000	461,000	(312,000)	(1,509,000)
Ending balance	157,392,000	179,028,000	157,392,000	179,028,000
Other comprehensive loss, tax			0
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			1,951,000
Other comprehensive loss, net of taxes of zero			(312,000)
Ending balance	1,639,000		1,639,000
Defined Benefit Pension Plan Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, 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 Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(608,000)	(9,841,000)	1,735,000	(7,871,000)
Ending balance	$ 1,423,000	$ (9,380,000)	$ 1,423,000	$ (9,380,000)