Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2017 USD ($) ft²	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Accrued termination charges		$ 0		$ 0		$ 0
Letter of credit issued		1,400,000		1,400,000		200,000
Rental expenses		400,000	$ 300,000	1,000,000	$ 900,000
Letter of credit issued, classified as long-term restricted cash		1,200,000		1,200,000		0
Ground lease expense		88,000		205,000
Liabilities related to indemnification agreements		0		0		$ 0
Building
Loss Contingencies [Line Items]
Construction in progress		6,200,000		6,200,000
Additional construction in progress		12,100,000		12,100,000
Capitalized interest costs during the construction period				100,000
Letter of Credit
Loss Contingencies [Line Items]
Letter of credit issued, classified as long-term restricted cash		1,200,000		$ 1,200,000
South San Francisco California Lease
Loss Contingencies [Line Items]
Lease expiration date				Apr. 30, 2021
Letter of credit expiration date				Dec. 31, 2017
South San Francisco California Lease | Letter of Credit
Loss Contingencies [Line Items]
Letter of credit issued		$ 200,000		$ 200,000
Westlake Village California Lease
Loss Contingencies [Line Items]
Lease expiration date				Apr. 30, 2019
Thousand Oaks California Lease
Loss Contingencies [Line Items]
Lease agreement area of office, lab and cellular therapy manufacturing space | ft²	90,580
Build-to-Suit Lease
Loss Contingencies [Line Items]
Lease initial term	15 years
Contractual obligations	$ 16,400,000
Lease extension term, option one	10 years
Lease extension term, option two	9 years