Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)			1 Months Ended			12 Months Ended
	Dec. 28, 2017 USD ($)	May 10, 2017 USD ($)	Jan. 31, 2014 USD ($) Program	Jun. 30, 2012 Targets	Jan. 31, 2012 USD ($) Targets	Dec. 31, 2017 USD ($) Product	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs | Program			2
Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement		$ 70,000,000
Potential amount eligible to receive for certain milestones		208,500,000
Milestone revenue receivable		475,000,000
Milestone payments received						$ 0
Number of products approved | Product						0
Royalty revenues						$ 0
Research program to develop laboratory research reagents						32 months
Deferred revenue						$ 53,000,000
Recognition of upfront fee						17,000,000
Bioverativ 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			115,800,000
Potential amount to be funded for achievement of specified commercialized and sales milestones			$ 160,500,000
Milestone payments received						$ 0
Number of products approved | Product						0
Royalty revenues						$ 0
Research program to develop laboratory research reagents						44 months
Deferred revenue						$ 4,600,000
Recognition of upfront fee						1,769,000	$ 2,321,000	$ 6,176,000
Shire AG [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement					$ 13,000,000
Deferred revenue						2,300,000
Recognition of upfront fee						$ 2,333,000	$ 2,181,000	2,167,000
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
Recognition of milestone									$ 1,000,000
Amount reimbursed and recognized related to prior obligations								$ 3,400,000
SB-525 [Member] | Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount to be funded for achievement of specified commercialized and sales milestones		266,500,000
C9ORF72 [Member] | Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement	$ 12,000,000
Maximum [Member] | SB-525 [Member] | Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		300,000,000
Maximum [Member] | Other Products [Member] | Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		$ 175,000,000
Maximum [Member] | C9ORF72 [Member] | Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount eligible to receive for certain milestones	60,000,000
Potential amount to be funded for achievement of specified commercialized and sales milestones	$ 90,000,000