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	Mar. 31, 2019 USD ($) Product Milestone	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues								$ 8,071,000	$ 12,637,000
Number of research programs | Program							2
Other long-term liabilities								1,911,000		$ 1,960,000
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								1,900,000		1,800,000
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		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 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
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								125,300,000		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								6,200,000	0
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Reimbursable Service Costs [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues								2,100,000	0
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								16,900,000		10,000,000
Revenues								$ 8,100,000
Number of products approved | Product								0
Pfizer [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									7,700,000
Revenue reduction due to decrease in measure of proportional cumulative performance								$ 3,000,000
Percentage of total revenue due to decrease in measure of proportional cumulative performance								38.00%
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								9,200,000		9,800,000
Revenues								$ 600,000	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 [Member] | 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 [Member] | 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
Sanofi [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
Sanofi [Member] | Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue								$ 3,800,000		$ 4,600,000
Sanofi [Member] | Collaboration and License Agreement [Member] | Royalty Revenues [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues								0
Sanofi [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
Sanofi [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
Bioverativ Inc [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues								1,949,000	4,382,000
Bioverativ Inc [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues								752,000	$ 1,154,000
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
Number of milestones included in transaction price | Milestone								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