Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Computer equipment and purchased software
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	3 years
Machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	3 years
Machinery and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Useful life of property and equipment	5 years