Other comprehensive income (loss) and accumulated other comprehensive items (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Other comprehensive income (loss) and accumulated other comprehensive items
Beginning balance		$ 156,437
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications	$ (227)	(2,341)
Amounts reclassified from other comprehensive items	0	0
Other comprehensive loss	(227)	(2,341)
Ending balance	386,090	386,090
Unrealized Gains/(Losses) On Marketable Securities, net of tax
Other comprehensive income (loss) and accumulated other comprehensive items
Beginning balance	(61)	22
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications	33	(50)
Amounts reclassified from other comprehensive items	0	0
Other comprehensive loss	33	(50)
Ending balance	(28)	(28)
Foreign Currency Translation
Other comprehensive income (loss) and accumulated other comprehensive items
Beginning balance	1,916	3,947
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other comprehensive income before reclassifications	(260)	(2,291)
Amounts reclassified from other comprehensive items	0	0
Other comprehensive loss	(260)	(2,291)
Ending balance	1,656	1,656
AOCI Attributable to Parent
Other comprehensive income (loss) and accumulated other comprehensive items
Beginning balance	1,855	3,969
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Ending balance	$ 1,628	$ 1,628