Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2014 USD ($) Program	Jun. 30, 2012 Targets	Jan. 31, 2012 USD ($) Targets	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Product	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs | Program	2
Research and development				$ 15,618,000	$ 13,460,000	$ 30,598,000	$ 25,543,000
Biogen Inc [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement	$ 20,000,000
Potential amount eligible to receive for certain milestones	126,300,000
Potential amount to be funded for achievement of specified commercialized and sales milestones	167,500,000
Milestone revenue receivable	293,800,000
Milestone payments received						$ 0
Number of products approved | Product						0
Royalty revenues						$ 0
Research program to develop laboratory research reagents						40 months
Deferred revenue				12,400,000		$ 12,400,000
Biogen Inc [Member] | Sickle cell disease [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable	7,500,000
Biogen Inc [Member] | Beta-thalassemia Project [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable	$ 7,500,000
Research and development				1,000,000	1,300,000	2,400,000	2,500,000
Biogen Inc [Member] | Other Projects [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development				1,000,000	300,000	$ 2,100,000	300,000
Shire AG [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement			$ 13,000,000
Potential amount eligible to receive for certain milestones			33,500,000
Potential amount to be funded for achievement of specified commercialized and sales milestones			$ 180,000,000
Number of products approved | Product						0
Royalty revenues						$ 0
Research program to develop laboratory research reagents						6 years
Deferred revenue				6,000,000		$ 6,000,000
Research and development				$ 4,200,000	$ 5,600,000	$ 8,700,000	$ 10,400,000
Aggregate number of gene targets | Targets			7
Number of initial gene targets | Targets			4
Number of gene targets | Targets		5
Number of additional gene targets | Targets		2
IND or CTA submission amount			$ 8,500,000
Milestone payment								$ 1,000,000