Income taxes - Net Deferred Tax Asset (Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Tax Credit Carryforward [Line Items]
Federal losses carryforward	$ 8.5	$ 10.7
State losses carryforward	17.4	18.1
Scientific research and experimental development credit carryforward	11.0	13.1
Stock compensation	7.6	7.5
Foreign NOLs	36.9	35.4
Deferred revenue	18.1	11.6
Inventory reserves	1.8	3.4
Lease liability	6.0
Other	7.5	4.9
Deferred tax asset	128.8	119.8
Fixed assets	(51.2)	(46.4)
Intangible assets	(54.5)	(60.4)
Right-of-use asset	(5.9)
Other	(3.2)	(0.7)
Deferred tax liability	(114.8)	(107.5)
Valuation allowance	(64.5)	(66.4)
Net deferred tax asset (liability)	(50.5)	(54.1)
Domestic Tax Authority
Tax Credit Carryforward [Line Items]
Federal losses carryforward	8.5
Research and development carryforward	9.0	10.1
Valuation allowance	(4.7)
Net deferred tax asset (liability)	(7.7)	(4.8)
State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
State losses carryforward	17.4
Research and development carryforward	5.0	$ 5.0
Valuation allowance	$ (16.4)