Major Customers, Partnerships and Strategic Alliances - California Institute for Regenerative Medicine - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	May. 31, 2014	May. 31, 2013	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research grants				$ 1,695	$ 1,990	$ 2,455
California Institute for Regenerative Medicine [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement	$ 5,600
Agreement to receive	4 years
Research grants				0	0	1,200
Collaboration agreement related costs and expenses				1,700	1,800	1,800
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
Agreement to receive		4 years
Research grants				1,200	1,400	100
Collaboration agreement related costs and expenses				$ 1,200	$ 1,100	$ 400
Unused funds received			$ 3,000