Right-of-use asset and lease liabilities - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended
	Nov. 30, 2018 ft² item	Mar. 31, 2019 USD ($)	Dec. 01, 2017	Jun. 30, 2016	Mar. 31, 2016	Nov. 30, 2013
Leases
Area of facility (in square feet) | ft²	30,655
Lexington
Leases
Lease term (in years)	5 years					10 years
Number of subsequent renewals | item	2
Renewal term (in years)	5 years
Minimum operating lease payments, Lease not yet commenced		$ 15.8
Lease term not yet commenced (in years)		10 years
Amsterdam
Leases
Lease term (in years)			10 years		16 years
Renewal term (in years)				5 years
Minimum rentals to be received		$ 9.2