Organization, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Organization And Summary Of Significant Accounting Policies [Line Items]
Liquid investments purchased with original maturities		three months or less
Accumulated deficit	$ (531,189)	$ (531,189)			$ (495,479)
Topic 606 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit				$ (494,362)
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	(5,163)	(5,163)		$ 1,117
Deferred revenues	(5,200)	(5,200)
ASU 2016-18 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Restricted cash balance	3,500	3,500
Pfizer [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Deferred revenues	31,900	31,900
Increase in recognition of up-front payment amount	1,600	$ 2,600
Pfizer [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues		47.00%
Bioverativ Inc [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Deferred revenues	6,200	$ 6,200
Reduction in recognition of up-front payment amount	$ 700	$ 1,500
Bioverativ Inc [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues		26.00%	48.00%
Shire AG [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Deferred revenues					$ 2,300
Shire AG [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues			11.00%