Major Customers, Partnerships and Strategic Alliances - California Institute for Regenerative Medicine - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	May 31, 2014	Oct. 31, 2009	May 31, 2013
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research grants	$ 820,000	$ 548,000
California Institute for Regenerative Medicine [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement			5,600,000	14,500,000
Expected amount to be received for research and development				5,200,000
Agreement to receive			4 years	4 years
Agreement date	Oct. 31, 2009
Research grants	0	0
Collaboration agreement related costs and expenses	300,000	0
California Institute for Regenerative Medicine [Member] | Beta-thalassemia [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement					6,400,000
Agreement to receive					4 years
Research grants	700,000	400,000
Collaboration agreement related costs and expenses	700,000	400,000
License agreement terms [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research grants	0	0
Collaboration agreement related costs and expenses	$ 200,000	$ 300,000