Available-for-Sale Marketable Securities - Schedule of Amortized Cost and Fair Values of Marketable Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Amortized Cost of marketable debt securities, contractual maturity, Less than one year	$ 146,363	$ 43,263
Amortized Cost of marketable debt securities, contractual maturity, One year to two years	21,449
Total Amortized Cost of marketable debt securities, contractual maturity	167,812	43,263
Fair value of marketable debt securities, contractual maturity, Less than one year	146,302	43,230
Fair value of marketable debt securities, contractual maturity,One year to two years	21,396
Total fair Value of marketable debt securities, contractual maturity	$ 167,698	$ 43,230

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