Marketable Securities and Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 484	$ 75,023
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		22,923
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	484	52,100
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents Measured at Fair Value on Recurring Basis	484	52,100
Money Market Funds [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents Measured at Fair Value on Recurring Basis	$ 484	52,100
Marketable securities [Member] | U.S. Treasury Bills and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		22,923
Marketable securities [Member] | Level 1 | U.S. Treasury Bills and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		$ 22,923