Stockholders' equity - Accumulated Other Comprehensive Income (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ (180,407,000)	$ (160,206,000)
Other comprehensive loss, net of taxes of zero	(2,343,000)	(1,970,000)
Ending balance	(169,658,000)	(167,238,000)
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	(2,343,000)
Ending balance	392,000
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
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,735,000)	7,871,000
Ending balance	$ 608,000	$ 9,841,000