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Fair value (Details) - USD ($)
Mar. 31, 2020
Dec. 31, 2019
Recurring    
Net liability arising from put/call options $ 825,000 $ 664,000
Fair Value, Inputs, Level 1 [Member]    
Recurring    
Net liability arising from put/call options 0 0
Fair Value, Inputs, Level 2 [Member]    
Recurring    
Net liability arising from put/call options 0 0
Fair Value, Inputs, Level 3 [Member]    
Recurring    
Net liability arising from put/call options $ 825,000 $ 664,000