Stockholders' equity - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ (160,206)	$ (160,240)
Other comprehensive loss, net of taxes of zero	1,970	(2,937)
Ending balance	(167,238)	(152,558)
Other comprehensive loss, tax	0
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	7,641
Other comprehensive loss, net of taxes of zero	1,970
Ending balance	9,611
Defined Benefit Pension Plan Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	230
Other comprehensive loss, net of taxes of zero	0
Ending balance	230
Accumulated other comprehensive loss
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	7,871	7,126
Ending balance	$ 9,841	$ 4,189