Intangible assets (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Intangible Assets Narrative Details
Gross value of intangible assets	$ 1,600,000	$ 1,600,000
Amortization expense	127,000	128,000	$ 132,000
Impairment of intangible assets	$ 0	$ 0	$ 40,000
Number of patents abandoned in prosecution	0	0	5