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Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2017 Sublease
Contingencies And Commitments [Line Items]
Cost sharing expensed relating to patent maintenance, defense and prosecution, incurred	$ 0.1	$ 0.2	$ 1.1	$ 1.0
Cost sharing expensed relating to patent maintenance, defense and prosecution, accrued			0.5		$ 2.8
Research and Office Facility [Member]
Contingencies And Commitments [Line Items]
Operating lease number of sublet portions \| Sublease						1
Operating leases, sublease payments receivable	$ 2.8		2.8
Research Agreements [Member]
Contingencies And Commitments [Line Items]
Upfront fees paid for license			1.2
Milestone payments			$ 0.6