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Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)		1 Months Ended								3 Months Ended								12 Months Ended
	Apr. 05, 2018 USD ($)	Sep. 30, 2019 USD ($)	Aug. 31, 2019 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2017 USD ($)	Jan. 31, 2014 USD ($) program	Jan. 31, 2012 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) milestone product option	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Agreement termination, term																		180 days
Cumulative-effect adjustment																						$ 897,000	$ 1,117,000
Revenues										$ 54,851,000	$ 21,958,000	$ 17,548,000	$ 8,071,000	$ 26,837,000	$ 23,562,000	$ 21,416,000	$ 12,637,000	$ 102,428,000	$ 84,452,000	$ 36,567,000
Other long-term liabilities										5,711,000				1,960,000				5,711,000	1,960,000
Accumulated Deficit
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Cumulative-effect adjustment																						$ 897,000	1,117,000
California Institute for Regenerative Medicine ("CIRM")
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Funds due under the agreement				$ 8,000,000.0
Other long-term liabilities										5,700,000				1,800,000				5,700,000	1,800,000
California Institute for Regenerative Medicine ("CIRM") \| Research grants
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		5,200,000
Kite Pharma, Inc. ("Kite")
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		34,350,000	25,517,000
Kite Pharma, Inc. ("Kite") \| Collaboration agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		$ 24,977,000	18,545,000
Kite Pharma, Inc. ("Kite") \| Collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received					$ 150,000,000.0
Initial research term of agreement																		6 years
Number of options to extend initial research term \| option																		2
Extended research term of agreement																		1 year
Separate fee for additional term																		$ 10,000,000.0
Collaborative arrangement estimated reimbursable service costs for new research plan		$ 3,400,000
Collaborative arrangement transaction price		189,300,000
Revenues under agreement		150,000,000.0
Collaborative arrangement estimated reimbursable service costs		$ 39,300,000
Deferred revenue										106,500,000				131,500,000				106,500,000	131,500,000
Kite Pharma, Inc. ("Kite") \| 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. ("Kite") \| 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. ("Kite") \| 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 SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price																		$ 105,000,000.0
Revenues under agreement							$ 70,000,000.0
Agreement termination, term																		15 years
Deferred revenue										4,000,000.0				10,000,000.0				$ 4,000,000.0	10,000,000.0
Revenues																		$ 39,359,000	37,810,000	17,008,000
Number of products approved \| product																		0
Research service fees																		$ 80,000,000.0
Pfizer SB-525 \| Topic 606
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred Revenue																							(5,200,000)
Pfizer SB-525 \| Topic 606 \| Accumulated Deficit
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Cumulative-effect adjustment																							5,200,000
Pfizer SB-525 \| SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of milestones included in transaction price \| milestone																		0
Pfizer SB-525 \| Collaboration agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues										8,700,000								$ 15,697,000	37,810,000	17,008,000
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress													$ 3,000,000.0
Pfizer SB-525 \| Maximum \| SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable							300,000,000.0
Potential amount to be funded for achievement of specified commercialized and sales milestones							266,500,000
Pfizer SB-525 \| Maximum \| Other products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable							175,000,000.0
Pfizer SB-525 \| 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 SB-525 \| 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 SB-525 \| Amended collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue										1,300,000								1,300,000
Milestone revenue receivable										25,000,000.0								25,000,000.0
Recognition of milestone																		$ 23,700,000
Pfizer C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received						$ 12,000,000.0
Agreement termination, term																		15 years
Pfizer C9ORF72 \| Collaboration agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		$ 1,827,000	2,188,000	0
Pfizer C9ORF72 \| C9ORF72
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement						12,000,000.0
Deferred revenue										8,000,000.0				9,800,000				8,000,000.0	9,800,000
Pfizer C9ORF72 \| 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 C9ORF72 \| 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
Sanofi
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement								$ 20,000,000.0
Revenues																		$ 22,680,000	13,516,000	12,258,000
Number of products approved \| product																		0
Number of research programs \| program								2
Sanofi \| Topic 606
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred Revenue																							(4,100,000)
Sanofi \| Topic 606 \| Accumulated Deficit
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Cumulative-effect adjustment																							$ 4,100,000
Sanofi \| Collaboration agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		$ 3,494,000	4,013,000	1,769,000
Sanofi \| Collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement transaction price																		89,200,000
Revenues under agreement																		20,000,000.0
Collaborative arrangement estimated reimbursable service costs																		55,700,000
Deferred revenue										1,700,000				$ 4,600,000				$ 1,700,000	4,600,000
Number of milestones included in transaction price \| milestone																		0
Recognition of milestone			$ 5,700,000															$ 7,100,000
Sanofi \| Collaboration and license agreement \| Milestone One
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received																		6,000,000.0
Milestone revenue receivable			$ 6,000,000.0
Sanofi \| Collaboration and license agreement \| Milestone Two
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable										7,500,000								7,500,000
Sanofi \| Collaboration and license agreement \| Milestone Three
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable										$ 13,500,000								13,500,000
Sanofi \| 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 \| 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
Shire AG
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues																		$ 0.0	$ 0.0	2,400,000
Shire AG \| Collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement									$ 13,000,000.0
Agreement termination, term																		90 days
Deferred revenue						$ 2,300,000														$ 2,300,000
Recognition of milestone																					$ 1,000,000.0