Long-term debt (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	May 06, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Long-term Debt
Interest rate (as a percent)	8.25%
Maximum borrowing capacity		$ 50.0		$ 50.0
Foreign currency loss		0.2		0.7
Foreign currency gain			$ 0.7		$ 2.3
Interest expense recorded		0.5	$ 0.6	1.4	$ 1.6
Venture debt loan facility with Hercules
Long-term Debt
Outstanding debt		20.0		$ 20.0		$ 20.0
Discount rate (as a percent)	5.25%
Back-end fee (as a percent)	4.85%
Facility fee (as a percent)	0.75%
Percentage of worldwide cash reserves				50.00%
Aggregate maturities of loan
Amortized cost net of discount and debt issuance costs		$ 20.7		$ 20.7		$ 20.8