BIAL Agreement - Additional Information (Detail) - B I A L - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
License Agreement [Line Items]
License agreement date	Feb. 09, 2017
Up-front license fees		$ (30)
Event Based Milestones
License Agreement [Line Items]
Milestone payables	$ 10
Regulatory Approval and Net Sales Milestones
License Agreement [Line Items]
Milestone payables	105
Regulatory and Clinical Results and FDA Acceptance Milestones
License Agreement [Line Items]
Milestone payables	$ 10