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Fair value measurements (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Recurring
Net asset arising from put/call options	$ 158,000	$ 280,000
Fair Value, Inputs, Level 1 [Member]
Recurring
Net asset arising from put/call options	0	0
Fair Value, Inputs, Level 2 [Member]
Recurring
Net asset arising from put/call options	0	0
Fair Value, Inputs, Level 3 [Member]
Recurring
Net asset arising from put/call options	$ 158,000	$ 280,000

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- References

No definition available.

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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