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Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)		1 Months Ended							3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 05, 2018 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Feb. 28, 2018	Dec. 31, 2017 USD ($)	May 31, 2017 USD ($)	Jan. 31, 2014 USD ($) Program	Jan. 31, 2012 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Product Milestone	Sep. 30, 2017 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 23,562,000	$ 11,812,000	$ 57,615,000	$ 23,490,000
Number of research programs \| Program							2
Other long-term liabilities									1,730,000		1,730,000
Research Grants [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									24,000	53,000	237,000	448,000
California Institute for Regenerative Medicine ("CIRM") [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement		$ 8,000,000
Other long-term liabilities									1,700,000		1,700,000
California Institute for Regenerative Medicine ("CIRM") [Member] \| Research Grants [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues											1,700,000
Kite Pharma, Inc. ("Kite") [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 9,028,000		$ 16,544,000
Kite Pharma, Inc. ("Kite") [Member] \| Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration waiting period under HSR expiration date at which time agreement became effective				Apr. 05, 2018
Milestone payments received			$ 150,000,000
Initial research term of agreement											6 years
Research agreement term											an option to extend the research term of the agreement for up to two additional one-year periods for a separate upfront fee of $10.0 million per year.
Lease agreement, extendable lease term									2 years		2 years
Separate upfront fee											$ 10,000,000
Collaborative arrangement transaction price											185,900,000
Revenues under agreement											150,000,000
Collaborative arrangement estimated reimbursable service costs											35,900,000
Deferred revenue									$ 137,800,000		137,800,000
Kite Pharma, Inc. ("Kite") [Member] \| Collaboration and License Agreement [Member] \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received	$ 3,010,000,000
Kite Pharma, Inc. ("Kite") [Member] \| Collaboration and License Agreement [Member] \| Achievement of Specified Research, Clinical Development, Regulatory and First Commercial Sale Milestones [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received	1,260,000,000
Kite Pharma, Inc. ("Kite") [Member] \| Collaboration and License Agreement [Member] \| Achievement of Specified Sales-based Milestones if Annual Worldwide Net Sales of Licensed Products Reach Specified Levels [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received	$ 1,750,000,000
Pfizer [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received											0
Revenues under agreement						$ 70,000,000
Deferred revenue									21,500,000		$ 21,500,000
Number of products approved \| Product											0
Pfizer [Member] \| SB-525 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of milestones included in transaction price \| Milestone											0
Pfizer [Member] \| Royalty Revenues [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues											$ 0
Pfizer [Member] \| C9ORF72 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement					$ 12,000,000
Deferred revenue									10,500,000		10,500,000
Revenues									400,000		$ 1,500,000
Number of milestones included in transaction price \| Milestone											0
Pfizer [Member] \| Maximum [Member] \| SB-525 [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
Milestone revenue receivable						300,000,000
Pfizer [Member] \| Maximum [Member] \| Other Products [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable						175,000,000
Pfizer [Member] \| Achievement of Specified Clinical Development Intellectual Property and Regulatory Milestones \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received						208,500,000
Pfizer [Member] \| Achievement of First Commercial Sale Milestones \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received						$ 475,000,000
Pfizer [Member] \| Achievement of Specified Preclinical Development Clinical Development and First Commercial Sale Milestones \| Maximum [Member] \| C9ORF72 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received					60,000,000
Pfizer [Member] \| Achievement of Commercial Milestones \| Maximum [Member] \| C9ORF72 [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received					90,000,000
Bioverativ Inc [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received											$ 0
Revenues under agreement							$ 20,000,000
Number of products approved \| Product											0
Revenues									2,240,000	3,401,000	$ 11,139,000	9,062,000
Bioverativ Inc [Member] \| Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price											75,700,000
Revenues under agreement											20,000,000
Collaborative arrangement estimated reimbursable service costs											55,700,000
Deferred revenue									$ 5,100,000		$ 5,100,000
Number of milestones included in transaction price \| Milestone											0
Bioverativ Inc [Member] \| Collaboration and License Agreement [Member] \| Royalty Revenues [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues											$ 0
Bioverativ Inc [Member] \| Collaboration and License Agreement [Member] \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received							276,300,000
Bioverativ Inc [Member] \| Collaboration and License Agreement [Member] \| Achievement Of Specified Clinical Development And Regulatory Milestones \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received							115,800,000
Bioverativ Inc [Member] \| Collaboration and License Agreement [Member] \| Achievement Of Specified Sales Milestones \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingent development - and sales-based milestone payments to be received							$ 160,500,000
Shire AG [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues										$ 1,172,000		$ 2,449,000
Shire AG [Member] \| Collaboration and License Agreement [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 milestone													$ 1,000,000