Cash and Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Restricted cash	$ 300	$ 300
Cash and Cash Equivalents [Line Items]
Amortized Cost	130,993	118,119
Unrealized Gain	21	0
Unrealized Loss	0	0
Estimated fair value	131,014	118,119
Commercial paper
Cash and Cash Equivalents [Line Items]
Amortized Cost	66,166	79,634
Unrealized Gain	0	0
Unrealized Loss	0	0
Estimated fair value	66,166	79,634
Corporate bonds
Cash and Cash Equivalents [Line Items]
Amortized Cost	12,667	16,119
Unrealized Gain	11	0
Unrealized Loss	0	0
Estimated fair value	12,678	16,119
U.S. Treasury and agency obligations
Cash and Cash Equivalents [Line Items]
Amortized Cost	52,160	22,366
Unrealized Gain	10	0
Unrealized Loss	0	0
Estimated fair value	52,170	22,366
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	20,732	38,022
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	4,489
Cash equivalents
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 25,221	$ 38,022

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. 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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