Summary of significant accounting policies and basis of presentation - Additional Information (Detail)			12 Months Ended
	Jun. 03, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment			1
Impairment losses			$ 0	$ 0	$ 0
Significant uncertain tax positions			$ 0	$ 0
Celgene Corporation [Member] | Up-front Payment Arrangement [Member] | Amended Collaborative Arrangement [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Collaboration agreement, cash payment received	$ 25,000,000
Pregenen [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Total purchase consideration		$ 30,991,000
Cash consideration		5,093,000
Common stock consideration		19,348,000
Contingent consideration		$ 6,550,000