Build-to-Suit Lease - Additional Information (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Operating Leased Assets [Line Items]
Operating lease payments		$ 19,410
Option to extend, existence, operating facility lease	true
Property and equipment, net		798	$ 265
Build-to-suit obligation		5,428
Build-to-suit related to construction costs incurred		3,500
Assets Held under Operating Leases [Member]
Operating Leased Assets [Line Items]
Property and equipment, net		8,900
1020 Marsh Road, Menlo Park, California [Member]
Operating Leased Assets [Line Items]
Operating facility lease area | ft²	34,500
Operating facility lease term	86 months
Lease agreement, additional extended lease term	5 years
Tenant improvement allowance	$ 1,900
Letter of credit to secure lease obligations	2,400
Operating lease payments	$ 20,000	$ 19,400
1060 Marsh Road [Member]
Operating Leased Assets [Line Items]
Operating facility lease area | ft²	17,400