Summary of Significant Accounting Policies - Additional Information (Detail) - Collaboration and License Agreement [Member] - GE Healthcare Limited [Member] - USD ($)		3 Months Ended	6 Months Ended
	Apr. 25, 2017	Jun. 30, 2017	Jun. 30, 2017
Schedule Of Accounting Policies [Line Items]
Upfront cash payment received	$ 5,000,000	$ 5,000,000	$ 5,000,000
Milestone revenues expected upon achievement of milestones	$ 60,000,000
Milestone revenues recognized under license agreement			$ 0