Major Customers, Partnerships and Strategic Alliances - California Institute for Regenerative Medicine - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Oct. 31, 2009 California Institute for Regenerative Medicine [Member]	May 31, 2013 California Institute for Regenerative Medicine [Member] Beta-thalassemia [Member]	Sep. 30, 2014 California Institute for Regenerative Medicine [Member] Beta-thalassemia [Member]	Sep. 30, 2014 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					$ 14,500,000	$ 6,400,000
Expected amount to be received for research and development					5,200,000
Agreement to receive					4 years	4 years
Agreement date					Oct. 31, 2009
Revenues under agreement	372,000	882,000	1,584,000	2,199,000			400,000	1,100,000
Collaboration agreement related costs and expenses							$ 400,000	$ 1,100,000