Summary of Significant Accounting Policies - Summary of Estimated Lives of Assets (Detail)	9 Months Ended
	Sep. 30, 2018
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	3 years
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	Lesser of lease term or 3 to 10 years
Minimum [Member] | Machinery, Manufacturing Equipment and Demonstration Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	3 years
Minimum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	3 years
Maximum [Member] | Machinery, Manufacturing Equipment and Demonstration Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	5 years
Maximum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful lives	10 years