Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Quarterly financial data (unaudited)
Operating expenses	$ 16,385	$ 16,664	$ 12,242	$ 10,907	$ 7,826	$ 10,137	$ 7,104	$ 5,879	$ 56,198	$ 30,946	$ 19,229
Net loss attributable to common stockholders	$ (15,789)	$ (16,189)	$ (11,139)	$ (9,508)	$ (6,656)	$ (7,673)	$ (6,461)	$ (10,044)	$ (52,625)	$ (30,834)	$ (19,702)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.41)	$ (0.46)	$ (0.35)	$ (0.30)	$ (0.29)	$ (0.24)	$ (0.26)	$ (2.02)	$ (1.54)	$ (1.33)	$ (3.96)