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Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)		1 Months Ended					3 Months Ended			6 Months Ended		7 Months Ended	13 Months Ended	25 Months Ended	65 Months Ended
	Apr. 05, 2018 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) milestone	May 31, 2017 USD ($)	Jan. 31, 2014 USD ($) program	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Milestone	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) Product	Jun. 30, 2019 USD ($) Product
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							$ 17,548,000		$ 21,416,000	$ 25,619,000	$ 34,053,000
Number of research programs \| program						2
Other long-term liabilities							3,288,000			3,288,000		$ 1,960,000	$ 3,288,000	$ 3,288,000	$ 3,288,000
California Institute For Regenerative Medicine Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement		$ 8,000,000.0
Other long-term liabilities							3,300,000			3,300,000		1,800,000	3,300,000	3,300,000	3,300,000
California Institute For Regenerative Medicine Agreement \| Research Grants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues												1,700,000	3,000,000.0
Kite Pharma Inc
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							9,060,000		7,515,000	17,372,000	7,515,000
Kite Pharma Inc \| Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							$ 6,227,000		5,953,000	12,386,000	5,953,000
Kite Pharma Inc \| Collaboration and License Agreement
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			$ 150,000,000.0
Initial research term of agreement	6 years
Lease agreement, extendable lease term	2 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.
Separate upfront fee	$ 10,000,000.0
Collaborative arrangement transaction price	185,900,000
Revenues under agreement	150,000,000.0
Collaborative arrangement estimated reimbursable service costs	35,900,000
Deferred revenue							$ 119,100,000			119,100,000		131,500,000	119,100,000	119,100,000	119,100,000
Kite Pharma Inc \| Collaboration and License Agreement \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received	3,010,000,000.00
Kite Pharma Inc \| Collaboration and License Agreement \| Achievement of Specified Research, Clinical Development, Regulatory and First Commercial Sale Milestones
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 \| Collaboration and License Agreement \| Achievement of Specified Sales-based Milestones if Annual Worldwide Net Sales of Licensed Products Reach Specified Levels
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
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received				$ 12,000,000.0
Revenues under agreement					$ 70,000,000.0									0
Deferred revenue							14,300,000			14,300,000		10,000,000.0	14,300,000	$ 14,300,000	14,300,000
Number of products approved \| Product														0
Revenues										25,600,000
Pfizer \| Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue reduction due to decrease in measure of proportional cumulative performance								$ 3,000,000.0
Revenues							455,000		617,000	$ 1,070,000	1,076,000
Pfizer \| SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of milestones included in transaction price \| Milestone										0
Pfizer \| Royalty Revenues
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues														$ 0
Pfizer \| C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue							8,700,000			$ 8,700,000		9,800,000	8,700,000	8,700,000	8,700,000
Number of milestones included in transaction price \| milestone				0
Pfizer \| Maximum \| SB-525
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					300,000,000.0
Pfizer \| Maximum \| Other Products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable					175,000,000.0
Pfizer \| Achievement of Specified Clinical Development Intellectual Property and Regulatory Milestones \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received					208,500,000
Pfizer \| Achievement of First Commercial Sale Milestones \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received					$ 475,000,000.0
Pfizer \| Achievement of Specified Preclinical Development Clinical Development and First Commercial Sale Milestones \| Maximum \| C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received				$ 60,000,000.0
Pfizer \| Achievement of Commercial Milestones \| Maximum \| C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received				$ 90,000,000.0
Pfizer SB-525 \| Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							4,635,000		8,183,000	1,595,000	15,841,000
Bioverativ Inc
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							3,059,000		4,516,000	5,009,000	8,898,000
Bioverativ Inc \| Collaboration Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues							901,000		$ 1,184,000	$ 1,654,000	$ 2,338,000
Bioverativ Inc \| Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price						$ 75,700,000
Revenues under agreement						20,000,000.0
Collaborative arrangement estimated reimbursable service costs						55,700,000
Number of milestones included in transaction price \| Milestone										0
Bioverativ Inc \| Collaboration and License Agreement \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received						276,300,000
Sanofi
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received															$ 0
Revenues under agreement						20,000,000.0
Number of products approved \| Product															0
Sanofi \| Collaboration and License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue							$ 2,900,000			$ 2,900,000		$ 4,600,000	$ 2,900,000	$ 2,900,000	$ 2,900,000
Sanofi \| Collaboration and License Agreement \| Royalty Revenues
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues															$ 0
Sanofi \| Collaboration and License Agreement \| Achievement Of Specified Clinical Development And Regulatory Milestones \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received						115,800,000
Sanofi \| Collaboration and License Agreement \| Achievement of Specified Sales Milestones \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received						$ 160,500,000