Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		0 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 08, 2014 Program	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 California Institute for Regenerative Medicine [Member]	Sep. 30, 2013 California Institute for Regenerative Medicine [Member]	Sep. 30, 2014 California Institute for Regenerative Medicine [Member]	Sep. 30, 2013 California Institute for Regenerative Medicine [Member]	Sep. 30, 2014 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Sep. 30, 2013 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Sep. 30, 2014 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Sep. 30, 2013 License agreement terms [Member] California Institute for Regenerative Medicine [Member]	Sep. 30, 2014 Biogen [Member] Product	Sep. 30, 2014 Biogen [Member]	Jan. 08, 2014 Biogen [Member]	Sep. 30, 2014 Biogen [Member] Beta-thalassemia [Member]	Sep. 30, 2014 Biogen [Member] Beta-thalassemia [Member]	Jan. 08, 2014 Biogen [Member] Beta-thalassemia [Member]	Jan. 08, 2014 Biogen [Member] Sickle cell disease [Member]	Sep. 30, 2014 Biogen [Member] Other Projects	Sep. 30, 2014 Biogen [Member] Other Projects	Jun. 30, 2012 Shire AG [Member] Targets	Jan. 31, 2012 Shire AG [Member] Targets	Sep. 30, 2014 Shire AG [Member]	Sep. 30, 2013 Shire AG [Member]	Sep. 30, 2014 Shire AG [Member] Product	Sep. 30, 2013 Shire AG [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs	2
Revenues under agreement	$ 20,000,000									$ 0	$ 500,000	$ 0	$ 1,200,000											$ 13,000,000
Milestone revenue receivable																293,800,000			7,500,000	7,500,000					213,500,000		213,500,000
Number of licensed products approved under agreement														0
Research program to develop laboratory research reagents															40 months												6 years
Deferred revenue														16,400,000	16,400,000										7,300,000		7,300,000
Research and development		16,290,000	8,703,000	41,753,000	26,201,000	500,000	500,000	1,500,000	1,400,000								2,000,000	4,500,000			500,000	800,000			5,100,000	3,500,000	15,500,000	10,300,000
Aggregate number of gene targets																								7
Number of initial gene targets																								4
Number of gene targets																							5
Number of additional gene targets																							2
IND or CTA submission amount																											8,500,000
Number of products approved																											0
Royalty revenue																											0
Milestone payment																									$ 1,000,000		$ 1,000,000