Fair value measurements - Changes in Fair Value of Liabilities-Classified as Stock Option (Details) - Liability classified stock options - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Liability at beginning of the period	$ 250	$ 253
Fair value of liability-classified options exercised in the period	0	(9)
Increase (decrease) in fair value of liability	(52)	(186)
Liability at end of the period	$ 198	$ 58