Cash Equivalents and Investments - Schedule of Fair Value of Securities, not Including Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 54,117	$ 73,427
Gross Unrealized Gains	3	3
Gross Unrealized Losses		(19)
Estimated Fair Value	54,120	73,411
Cash equivalents	23,613	15,091
Short-term investments	30,507	58,320
Total cash equivalents and investments	54,120	73,411
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	23,613	10,606
Estimated Fair Value	23,613	10,606
U.S. Government Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,476	9,976
Gross Unrealized Gains	1
Gross Unrealized Losses		(1)
Estimated Fair Value	3,477	9,975
Corporate Notes
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,284	16,514
Gross Unrealized Gains	2	3
Gross Unrealized Losses		(18)
Estimated Fair Value	11,286	16,499
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,744	36,331
Estimated Fair Value	$ 15,744	$ 36,331

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).

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

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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Amount of 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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