Fair Value of Financial Instruments - Schedule of fair value hierarchy for financial assets (cash equivalents and investments) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,755	$ 632
Corporate notes and commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,072
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,755	$ 632
Fair Value, Measurements, Recurring [Member] | Corporate notes and commercial paper [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,072