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Fair value measurements (Details) - USD ($) | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Recurring |  |
Net asset arising from put/call options | $ 233,000 | $ 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 | $ 233,000 | $ 280,000

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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 + Details
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