Summary of Significant Accounting Policies - Additional Information (Detail) - Collaboration and License Agreement - GE Healthcare Limited - USD ($)		9 Months Ended
Apr. 25, 2017	Sep. 30, 2017
Schedule Of Accounting Policies [Line Items]
Upfront cash payment received	$ 5,000,000	$ 5,000,000
Milestone revenues expected upon achievement of milestones	$ 60,000,000
Milestone revenues recognized under license agreement		$ 0

The amount of consideration recognized during the period for the milestone or milestones.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 28
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=68058744&loc=SL6892177-166501

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration