Organization, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
Organization And Summary Of Significant Accounting Policies [Line Items]
Revenues	$ 8,071	$ 12,637
Increase in net loss	$ 42,203	$ 20,187
Liquid investments purchased with original maturities	three months or less
Non-current restricted cash	$ 3,500		$ 3,500
Accumulated deficit	(603,949)		(562,696)
Topic 606 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit					$ 1,100
Topic 842 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit				$ (561,799)
Topic 842 [Member] | Adjustments Due to the Adoption of ASC Topic 842 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit				$ 897
Intangible Assets [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Reclassification adjustment			(600)
Other Noncurrent Assets [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Reclassification adjustment			$ 600
Pfizer [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Revenues	$ 8,100
Pfizer [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues		61.00%
Kite Pharma, Inc. ("Kite") [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues	75.00%
Bioverativ Inc [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Revenues	$ 1,949	$ 4,382
Bioverativ Inc [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues	18.00%	35.00%
Collaboration Agreement [Member] | Pfizer [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Revenues	$ (3,000)
Increase in net loss	$ 3,000
Increase in basic net loss per share	$ 0.03