Stockholders' equity - Accumulated Other Comprehensive Income (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 147,984,000	$ 169,458,000	$ 160,240,000	$ 194,451,000
Other comprehensive loss, net of taxes of zero	(4,615,000)	(5,588,000)	(3,468,000)	(7,967,000)
Ending balance	149,997,000	161,174,000	149,997,000	161,174,000
Other comprehensive loss, tax			0
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(6,897,000)
Other comprehensive loss, net of taxes of zero			(3,468,000)
Ending balance	(10,365,000)		(10,365,000)
Defined Benefit Pension Plan Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(229,000)
Other comprehensive loss, net of taxes of zero			0
Ending balance	(229,000)		(229,000)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(5,979,000)	(1,981,000)	(7,126,000)	398,000
Ending balance	$ (10,594,000)	$ (7,569,000)	$ (10,594,000)	$ (7,569,000)