Fair Value Measurements and Investments - Schedule of Reconciliation of Debt Security and Warrant (Detail) - eNeura Inc [Member] - Debt Security and Warrant [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 0	$ 17,820
Gains or losses recorded for the period
Recognized in other comprehensive income (loss)	0	(2,593)
Change in classification of debt security to held to maturity	0	(15,227)
Issuance of Warrant as consideration for extension	0	491
Ending balance	$ 0	$ 491