Subsequent events - Additional Information (Details) $ / shares in Units, $ in Thousands	May 11, 2020 USD ($) Warrant $ / shares shares	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2018 USD ($)
Subsequent Event [Line Items]
Total agreed amount of royalty purchase price		$ 64,346	$ 43,251
Amended Royalty Agreements | Subsequent Event
Subsequent Event [Line Items]
Outstanding amount of royalty agreement	$ 37,500
Total agreed amount of royalty purchase price	$ 117,500
Number of warrants issued | Warrant	2
Class of warrant, shares issued | shares	550,661
Common stock, exercise price per share | $ / shares	$ 13.62
Amended Royalty Agreements | Subsequent Event | Fifth Tranche
Subsequent Event [Line Items]
Outstanding amount of royalty agreement	$ 20,000
Royalty Agreements
Subsequent Event [Line Items]
Outstanding amount of royalty agreement		$ 20,000
Total agreed amount of royalty purchase price				$ 80,000