Stockholders' equity - Accumulated Other Comprehensive Income (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Stockholders' Equity Note [Abstract]
Other comprehensive income (loss), tax	$ 0
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	160,240,000	$ 194,451,000
Other comprehensive income (loss), net of taxes of zero and reclassifications	2,937,000	6,103,000
Ending balance	152,558,000	183,720,000
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(6,897,000)
Other comprehensive income (loss), net of taxes of zero and reclassifications	2,937,000
Ending balance	(3,960,000)
Defined Benefit Pension Plan Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(229,000)
Other comprehensive income (loss), net of taxes of zero and reclassifications	0
Ending balance	(229,000)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(7,126,000)	398,000
Ending balance	$ (4,189,000)	$ 6,501,000