Revenue recognition - Future performance obligations (Details) $ in Millions	Sep. 30, 2019 USD ($)
Transaction Price Allocated to Future Performance Obligations
Amount of transaction price allocated to performance obligations	$ 5.3
Service-type warranties and research services
Transaction Price Allocated to Future Performance Obligations
Amount of transaction price allocated to performance obligations	2.8
Undelivered licenses of intellectual property
Transaction Price Allocated to Future Performance Obligations
Amount of transaction price allocated to performance obligations	1.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Transaction Price Allocated to Future Performance Obligations
Amount of transaction price allocated to performance obligations	$ 4.9
Revenue recognition period for remaining performance obligation	12 months