Organization, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	$ (514,549)			$ (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	(2,863)		1,117
Deferred revenues			$ (1,100)
ASU 2016-18 [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Restricted cash balance	3,500
Pfizer [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Deferred revenues	40,100
Increase in recognition of up-front payment amount	$ 5,200
Pfizer [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues	61.00%
Bioverativ Inc [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Deferred revenues	$ 7,500
Reduction in recognition of up-front payment amount	$ 4,100
Bioverativ Inc [Member] | Customer Concentration Risk [Member] | Total revenues [Member]
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenues	35.00%	63.00%
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		20.00%