Financial instruments (Details) - USD ($)		Dec. 31, 2018	Dec. 31, 2017
Recurring
Net asset arising from put/call options	[1]	$ 24,000	$ 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		$ 24,000	$ 280,000

[1] The net asset arising from the put/call options is included in other assets in the accompanying consolidated balance sheets as of December 31, 2018 and 2017.