Stockholders' equity - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 152,558	$ 183,720	$ 160,240	$ 194,451
Other comprehensive income (loss), net of taxes of zero and reclassifications	(1,790)	(8,482)	1,147	(2,379)
Ending balance	147,984	169,458	147,984	169,458
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(6,897)
Other comprehensive income (loss), net of taxes of zero and reclassifications			1,147
Ending balance	(5,750)		(5,750)
Defined Benefit Pension Plan Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(229)
Other comprehensive income (loss), net of taxes of zero and reclassifications			0
Ending balance	(229)		(229)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,189)	6,501	(7,126)	398
Ending balance	$ (5,979)	$ (1,981)	$ (5,979)	$ (1,981)