Deferred revenue and remaining performance obligations - Performance Obligation (Details) $ in Millions	Jun. 30, 2019 USD ($)
Revenue from Contract with Customer [Abstract]
Remaining performance obligation, amount	$ 70.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation, remaining duration (in years)	5 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-07-01 | Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligation, remaining duration (in years)	8 years