Right-of-use asset and lease liabilities - Narrative (Details) € in Millions, $ in Millions	1 Months Ended	6 Months Ended
	Nov. 30, 2018 item	Jun. 30, 2020 USD ($)	Jun. 30, 2020 EUR (€)	Dec. 31, 2019	Jun. 30, 2019 ft²	Dec. 01, 2017	Jun. 30, 2016	Mar. 31, 2016
Leases
Area of facility (in square feet) | ft²					30,655
Weighted-average remaining lease term, Operating leases		9 years 9 months 18 days	9 years 9 months 18 days	10 years 3 months 18 days
Weighted-average discount rate, Operating leases		11.33%	11.33%	11.33%
Lexington
Leases
Lease term (in years)	5 years
Number of subsequent renewals | item	2
Renewal term (in years)	5 years
Amsterdam
Leases
Lease term (in years)						10 years		16 years
Renewal term (in years)							5 years
Minimum rentals to be received		$ 6.4	€ 5.7