Stockholders' equity - Restricted stock activity (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Shares
Beginning balance (in shares)	39,900
Granted (in shares)	100,319
Vested (in shares)	(51,201)
Ending balance (in shares)	89,018
Weighted average grant date fair value
Beginning balance (in dollars per share)	$ 2.31
Granted (in dollars per share)	1.88
Vested (in dollars per share)	2.07
Ending balance (in dollars per share)	$ 1.97
Expense related to restricted stock awards	$ 122	$ 184
Future expected expense related to restricted stock awards	$ 159
Period of recognition	2 years 10 months 24 days

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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