Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail) (USD $)	0 Months Ended			3 Months Ended					1 Months Ended		3 Months Ended		0 Months Ended	3 Months Ended
	Jan. 08, 2014 Program	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 California Institute for Regenerative Medicine [Member]	Mar. 31, 2013 California Institute for Regenerative Medicine [Member]	Mar. 31, 2014 Beta-thalassemia [Member] California Institute for Regenerative Medicine [Member]	Mar. 31, 2014 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Mar. 31, 2013 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Jun. 30, 2012 Shire AG [Member] Targets	Jan. 31, 2012 Shire AG [Member] Targets	Mar. 31, 2014 Shire AG [Member] Product	Mar. 31, 2013 Shire AG [Member]	Jan. 08, 2014 Biogen [Member]	Mar. 31, 2014 Biogen [Member]	Jan. 08, 2014 Biogen [Member] Beta-thalassemia [Member]	Jan. 08, 2014 Biogen [Member] Sickle cell disease [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs	2
Upfront license fee										$ 13,000,000			$ 20,000,000
Milestone payments													293,800,000		7,500,000	7,500,000
Research program to develop laboratory research reagents											6 years			40 months
Deferred revenue		8,548,000	2,282,000								8,400,000			19,300,000
Aggregate number of gene targets										7
Number of initial gene targets										4
Number of gene targets									5
Number of additional gene targets									2
Potential amount to be funded for certain Shire milestones											213,500,000
IND or CTA submission amount											8,500,000
Number of products approved											0
Royalty revenue											0
Collaboration agreement related costs and expenses				300,000	400,000	1,300,000					4,800,000	3,100,000
Revenues under agreement						$ 400,000	$ 0	$ 400,000