Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets
Available-for-sale marketable securities	$ 141,952	$ 167,698
Recurring [Member]
Financial assets
Total financial assets	156,909	183,548
U.S. Government Agency Obligations [Member]
Financial assets
Available-for-sale marketable securities	12,447	17,859
U.S. Government Agency Obligations [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	12,447	17,859
Corporate Bonds [Member]
Financial assets
Available-for-sale marketable securities	69,960	75,910
Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	69,960	75,910
Municipal Bonds [Member]
Financial assets
Available-for-sale marketable securities	5,500	4,000
Municipal Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	5,500	4,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Total financial assets	14,957	15,850
Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Total financial assets	141,952	167,698
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government Agency Obligations [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	12,447	17,859
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	69,960	75,910
Significant Other Observable Inputs (Level 2) [Member] \| Municipal Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	5,500	4,000
Money Market Funds [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	14,188	15,081
Restricted cash	769	769
Money Market Funds [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	14,188	15,081
Restricted cash	769	769
U.S. Treasury Securities [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	15,937	19,539
U.S. Treasury Securities [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	15,937	19,539
Commercial Paper [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	38,108	50,390
Commercial Paper [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	$ 38,108	$ 50,390

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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