Inventories - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017 principal_inventory_category
Property, Plant and Equipment [Line Items]
Number of principal categories of inventory	3
Demonstration inventory shelf life (in years)	3 years
Refurbished demonstration inventory selling period (in months)	12 months
Service Inventory
Property, Plant and Equipment [Line Items]
Inventory, remaining useful life (in years)	3 years