Subsequent events (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 07, 2020	Mar. 31, 2020	Mar. 31, 2019	Jul. 02, 2018
Subsequent events
Net proceeds from offering of common stock			$ 92,162
ATM
Subsequent events
Amount of shares authorized to be offered and sold				$ 75,000
Issuance of common stock in offering (in shares)		0
Subsequent event | ATM
Subsequent events
Net proceeds from offering of common stock	$ 65,153
Subsequent event | ATM, $5.59 per share
Subsequent events
Issuance of common stock in offering (in shares)	8,938,599
Share price (in dollars per share)	$ 5.59
Subsequent event | ATM, $7.74 per share
Subsequent events
Issuance of common stock in offering (in shares)	1,962,000
Share price (in dollars per share)	$ 7.74