Cash Equivalents and Investments - Schedule of Fair Value of Securities, not Including Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 73,427	$ 102,325
Gross Unrealized Gains	3
Gross Unrealized Losses	(19)	(65)
Estimated Fair Value	73,411	102,260
Cash equivalents	15,091	5,574
Short-term investments	58,320	93,692
Long-term investments		2,994
Total	73,411	102,260
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	10,606	5,574
Estimated Fair Value	10,606	5,574
U.S. Government Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,976	24,969
Gross Unrealized Losses	(1)	(29)
Estimated Fair Value	9,975	24,940
Corporate Notes
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	16,514	24,539
Gross Unrealized Gains	3
Gross Unrealized Losses	(18)	(36)
Estimated Fair Value	16,499	24,503
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	36,331	47,243
Estimated Fair Value	$ 36,331	$ 47,243

Cash equivalents and current and noncurrent investments.

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

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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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