Stockholders' equity (deficit) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Vote $ / shares shares	Jun. 30, 2020 USD ($)	Dec. 31, 2020 $ / shares shares	Nov. 12, 2020 USD ($)	Sep. 30, 2019 $ / shares
Stockholders' equity
Aggregate number of authorized shares for all classes of equity | shares	440,000,000		440,000,000
Common stock, shares authorized | shares	400,000,000		400,000,000		400,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001		$ 0.001		$ 0.001
Preferred stock, shares authorized | shares	40,000,000		40,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001
Number of voting rights per common share | Vote			1
Maximum amount to be issued under universal shelf registration statement						$ 350,000,000.0
Maximum amount to be issued under at-the-market offering program						$ 50,000,000.0
Amount of equity or debt securities sold under at-the-market offering program	$ 0		$ 0
Stock based compensation expense	$ 1,863,498	$ 1,256,460	$ 3,686,060	$ 2,161,931