Fair Value Measurements and Investments - Schedule of Reconciliation of Debt Security and Warrant (Detail) - eNeura Inc [Member] - Debt Security And Warrant - Fair Value, Inputs, Level 3 [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 17,820
Gains or losses recorded for the period
Recognized in other comprehensive income (loss)	(2,593)
Change in classification of debt security to held to maturity	(15,227)
Issuance of Warrant as consideration for prior extension of debt maturity date	491
Impairment of warrant	$ (491)