Income taxes - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Taxes
Income tax expense	$ 0	$ 0	$ 0	$ 0
Uncertain tax positions	$ 800,000		$ 800,000		$ 800,000