Long-term debt (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Dec. 06, 2018	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	May 31, 2020	Dec. 31, 2019
2016 Amended Facility | Hercules | Second Amended and Restated Loan and Security Agreement [Member]
Long-term Debt
Outstanding debt	$ 20.0
Back-end fees due						$ 1.0
2018 Amended Facility | Hercules
Long-term Debt
Aggregate amount of Equity Financing				$ 90.0
Foreign currency loss					$ 0.2
Foreign currency gain		$ 0.7	$ 0.5	0.0
2018 Amended Facility | Hercules | Second Amended and Restated Loan and Security Agreement [Member]
Long-term Debt
Outstanding debt	$ 35.0	35.0		35.0			$ 35.0
Interest rate (as a percent)	8.85%
Back-end fee (as a percent)	4.95%
Borrowing capacity subject to discretion		15.0		$ 15.0
Additional unconditional borrowing capacity	$ 15.0
Proceeds from loan increment	$ 15.0
Facility fee (as a percent)	0.50%
2018 Amended Facility | Prime Rate | Hercules | Second Amended and Restated Loan and Security Agreement [Member]
Long-term Debt
Variable interest rate basis	8.85%
Discount rate (as a percent)	5.50%
Venture debt loan facility | 2018 Amended Facility
Long-term Debt
Minimum cash and cash equivalents in U.S. bank accounts				65.00%
Assets pledged to secure facilities by directly or indirectly				$ 385.3
Assets not being pledged to secure facilities by directly or indirectly				110.0
Aggregate maturities of loan
Long-term debt		35.6		35.6			$ 36.3
Venture debt loan facility | 2018 Amended Facility | Hercules
Long-term Debt
Interest expense recorded		$ 0.9	$ 0.9	$ 1.8	$ 1.9