Major Customers, Partnerships and Strategic Alliances - Additional Information (Detail)		1 Months Ended							3 Months Ended		6 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 ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Product Milestone	Jun. 30, 2017 USD ($)	Dec. 31, 2014 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 21,416,000	$ 8,253,000	$ 34,053,000	$ 11,678,000
Number of research programs | Program							2
Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									21,289,000	7,977,000	33,840,000	11,283,000
Revenue from Grants [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									127,000	276,000	213,000	395,000
California Institute for Regenerative Medicine ("CIRM") [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue									1,700,000		1,700,000
Funds due under the agreement		$ 8,000,000
California Institute for Regenerative Medicine ("CIRM") [Member] | Revenue from Grants [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 0		$ 0
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
Deferred revenue									$ 144,000,000		144,000,000
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Upfront License Fee [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement											185,900,000
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
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									1,500,000
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									6,000,000
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									31,900,000		$ 31,900,000
Number of products approved | Product											0
Pfizer [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount eligible to receive for certain milestones						208,500,000
Milestone revenue receivable						475,000,000
Pfizer [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									8,100,000		$ 15,800,000
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] | SB-525 [Member] | Maximum [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] | Other Products [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable						$ 175,000,000
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									10,900,000		10,900,000
Revenues									600,000		$ 1,100,000
Number of milestones included in transaction price | Milestone											0
Pfizer [Member] | C9ORF72 [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount eligible to receive for certain milestones					60,000,000
Potential amount to be funded for achievement of specified commercialized and sales milestones					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				75,700,000
Deferred revenue									6,200,000		6,200,000
Revenues									4,516,000	3,510,000	$ 8,898,000	5,661,000
Number of products approved | Product											0
Number of milestones included in transaction price | Milestone											0
Bioverativ Inc [Member] | Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Potential amount eligible to receive for certain milestones							115,800,000
Potential amount to be funded for achievement of specified commercialized and sales milestones							$ 160,500,000
Bioverativ Inc [Member] | Research Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									3,332,000	3,068,000	$ 6,560,000	4,777,000
Bioverativ Inc [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues									$ 1,184,000	442,000	2,338,000	884,000
Bioverativ Inc [Member] | Royalty Revenues [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues											$ 0
Shire AG [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement								$ 13,000,000
Deferred revenue					$ 2,300,000
Revenues										586,000		1,277,000
Recognition of milestone													$ 1,000,000
Shire AG [Member] | Research Services [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues										3,000		110,000
Shire AG [Member] | Collaboration Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues										$ 583,000		$ 1,167,000