Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended			6 Months Ended
	May 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Mar. 31, 2017 USD ($) ft² Lease	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Number of lease renewal option | Lease			1
Operating lease payments				$ 18,385
Inducement payment received				1,896
Build-to-suit obligation				7,324	$ 5,428
Property and equipment, net				680	$ 798
Build-to-suit related to construction costs incurred				3,500
Other potential commitments				10,000
Maximum [Member]
Loss Contingencies [Line Items]
Commitment to make development and sales-related milestone payments				30,000
Assets Held under Operating Leases [Member]
Loss Contingencies [Line Items]
Property and equipment, net				8,800
1020 Marsh Road, Menlo Park, California [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²			34,500
Lease facility delivery date			2017-04
Lease commencement date			August 8, 2017
Operating facility lease term			86 months
Lease agreement, one renewal option term			5 years
Operating lease payments			$ 20,000	$ 18,400
Tenant improvement allowance			1,900
Letter of credit to secure lease obligations			$ 2,400
Tenant improvement allowance received	$ 400	$ 1,500
Percentage of tenant improvement allowance received	20.00%	80.00%
Lease facility began occupying date			2017-08
Lease agreement, additional extended lease term			5 years
1060 Marsh Road [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²			17,400