Subsequent events (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Jan. 14, 2020	Dec. 31, 2019	Dec. 31, 2018
Subsequent Event [Line Items]
Granted		2,955,000	2,515,000
Subsequent Event | Sales Agreement
Subsequent Event [Line Items]
New shares issued under the agreement	11,691,666
Public offering price of the shares sold	$ 3.00
Net proceeds from issuance of common stock	$ 33.3