Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Number of shares issued	3,162,500
Allocated share-based compensation expense		$ 3,874	$ 2,223	$ 7,534	$ 3,989
Sale of stock, price per share	$ 38.85	$ 38.85		$ 38.85
Exercise price of warrants (in dollars per share)	$ 0.00	$ 0.00		$ 0.00
Consideration received on transaction	$ 130,700
Shares issued		110,860	0	513,808
Proceeds from stock options exercised		$ 1,500		$ 7,400
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		1,771	$ 938	3,264	1,919
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		$ 2,103	$ 1,285	$ 4,270	$ 2,070
Note Warrant
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Number of shares issued	421,650
Sale of stock, price per share	$ 38.85	$ 38.85		$ 38.85
Ownership Threshold One
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Warrant exercise threshold, percentage of common stock owned				9.99%
Ownership Threshold Two
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Warrant exercise threshold, percentage of common stock owned				15.00%
Ownership Threshold Three
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Warrant exercise threshold, percentage of common stock owned				19.99%