Fair value measurement - Estimated Carrying Amount and Fair Value of Financial Instruments Measured and Recorded at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Liabilities:
Derivative liability	$ 663	$ 0
Assets:
Derivative asset	0	258
Fair Value, Measurements, Recurring
Liabilities:
Derivative liability	663
Assets:
Derivative asset		258
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Liabilities:
Derivative liability	0
Assets:
Derivative asset		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Liabilities:
Derivative liability	663
Assets:
Derivative asset		258
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Liabilities:
Derivative liability	$ 0
Assets:
Derivative asset		$ 0