Common stock - Equity offerings (Details) - Common stock - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Jun. 14, 2018	May 23, 2018	May 16, 2018	Dec. 31, 2020
Common stock
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs (in shares)				46,511,628
Cantor | Follow-on offerings | Underwriting Agreement
Common stock
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs (in shares)		1,166,666
Net proceeds from issuance of stock		$ 38.3
Options are exercisable from date of prospectus supplement			30 days
Cantor | Follow-on offerings | Underwriting Agreement | Plan
Common stock
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs (in shares)			7,777,778
Share price (in dollars per share)			$ 4.31
Consonance | Follow-on offerings | Purchase Agreement
Common stock
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs (in shares)	7,166,666
Share price (in dollars per share)	$ 6.00
Net proceeds from issuance of stock	$ 42.9