Fair value measurements (Details 1)	6 Months Ended
	Jun. 30, 2018 USD ($)	[1]
Net asset arising from put/call options
Balance as of December 31, 2017	$ 280,000
Unrealized loss	(122,000)
Balance as of June 30, 2018	$ 158,000

[1] In connection with the Company’s acquisition of 51% of Aerex in February 2016, the Company acquired from Aerex’s former sole shareholder an option to compel such shareholder to sell, and granted to such shareholder an option to require the Company to purchase, the shareholder’s remaining 49% ownership interest in Aerex at a price based upon the fair market value of Aerex at the time of the exercise of the option. The options are exercisable on or after the third anniversary of the February 2016 acquisition date. The net asset arising from the put/call options is included in other assets in the accompanying condensed consolidated balance sheets as of June 30, 2018 and December 31, 2017.