Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Transfers
Fair value of assets transferred from Level 1 to Level 2	$ 0	$ 0
Fair value of assets transferred from Level 2 to Level 1	0	0
Fair value of assets transferred from Level 1 to Level 3	0	0
Fair value of assets transferred from Level 2 to Level 3	0	0
Fair value of assets transferred from Level 3 to Level 1	0	0
Fair value of assets transferred from Level 3 to Level 2	0	0
U.S. Treasury Notes
Assets
Amortized cost of short-term investments	186,415	235,332
Unrealized gain (loss) on short-term investments	37	(4)
Short-term investments	186,452	235,328
Fair Value \| Recurring basis
Assets
Assets	222,061	280,893
Fair Value \| Recurring basis \| Money Market Funds
Assets
Restricted cash	210	210
Cash equivalents	25,207	29,721
Fair Value \| Recurring basis \| U.S. Treasury Notes
Assets
Cash equivalents	10,192	15,634
Short-term investments	186,452	235,328
Fair Value \| Recurring basis \| Level 1
Assets
Assets	25,417	29,931
Fair Value \| Recurring basis \| Level 1 \| Money Market Funds
Assets
Restricted cash	210	210
Cash equivalents	25,207	29,721
Fair Value \| Recurring basis \| Level 2
Assets
Assets	196,644	250,962
Fair Value \| Recurring basis \| Level 2 \| U.S. Treasury Notes
Assets
Cash equivalents	10,192	15,634
Short-term investments	$ 186,452	$ 235,328

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 3.

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Amount of transfers of assets measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 1.

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Amount of transfers of assets measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 2.

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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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Amortized cost 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 restricted as to withdrawal or usage. 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.

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