Intangible assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Identifiable intangible assets [Abstract]
Gross amount	$ 3,667	$ 3,667
Accumulated amortization	(3,122)	(2,779)
Amortization expense	343	454	$ 515
Future amortization expense [Abstract]
2016	236
2017	98
2018	98
2019	82
2020	31
2021	0
Purchased Technology [Member]
Identifiable intangible assets [Abstract]
Gross amount	1,604	1,604
Accumulated amortization	(1,433)	(1,357)
Customer Relationships [Member]
Identifiable intangible assets [Abstract]
Gross amount	1,300	1,300
Accumulated amortization	(1,163)	(946)
Trademarks [Member]
Identifiable intangible assets [Abstract]
Gross amount	480	480
Accumulated amortization	(257)	(210)
Covenant Not to Compete [Member]
Identifiable intangible assets [Abstract]
Gross amount	146	146
Accumulated amortization	(146)	(146)
Patents [Member]
Identifiable intangible assets [Abstract]
Gross amount	57	57
Accumulated amortization	(43)	(40)
Other [Member]
Identifiable intangible assets [Abstract]
Gross amount	80	80
Accumulated amortization	$ (80)	$ (80)