Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)		1 Months Ended							3 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 ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Product Milestone	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 26,837,000	$ 23,562,000	$ 21,416,000	$ 12,637,000	$ 13,077,000	$ 11,812,000	$ 8,253,000	$ 3,425,000	$ 84,452,000	$ 36,567,000	$ 19,389,000
Number of research programs | Program							2
Other long-term liabilities									1,960,000								1,960,000
Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	84,065,000	35,960,000	18,881,000
Research Grants [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	387,000	607,000	508,000
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									1,800,000								1,800,000
Interest payable									$ 100,000								100,000
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] | 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 for up to two additional one-year periods for a separate fee of $10.0 million per year.
Lease agreement, extendable lease term									2 years								2 years
Separate fee for additional term																	$ 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									$ 131,500,000								131,500,000
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	18,500,000
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Research Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	7,000,000
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
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									10,000,000								$ 10,000,000
Number of products approved | Product																	0
Pfizer [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	$ 37,800,000	17,000,000
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									9,800,000								9,800,000
Revenues																	$ 2,200,000
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]
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]
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]
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]
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
Revenues																	$ 13,516,000	12,258,000	8,886,000
Number of products approved | Product																	0
Bioverativ Inc [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	$ 4,013,000	1,769,000	2,321,000
Bioverativ Inc [Member] | Research Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																	9,503,000	10,489,000	6,565,000
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									$ 4,600,000								$ 4,600,000
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]
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 [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
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 [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
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																	$ 0	2,400,000	$ 3,300,000
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								$ 2,300,000					$ 2,300,000
Recognition of milestone																				$ 1,000,000