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Commitments and Contingencies - Rent expense and purchase obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rent expense and inventory purchase commitments
Aggregate rent expense under operating leases	$ 4.9	$ 3.8	$ 3.4
Purchase obligations	34.3
Inventories
Rent expense and inventory purchase commitments
Purchase obligations expected to be consummated in the next 12 months	18.0
Hosting arrangements
Rent expense and inventory purchase commitments
Purchase obligations will be used in the next 3 years	14.1
Other software agreements
Rent expense and inventory purchase commitments
Purchase obligations will be used in one to two years	$ 2.2