Cash Equivalents and Marketable Securities - Schedule of Estimated Fair Value of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	$ 12,800	$ 8,924
Cash equivalents, estimated fair value	12,800	8,924
Marketable securities, amortized cost	50,236	37,584
Marketable securities, gross unrealized gains	2
Marketable securities, gross unrealized losses	(35)	(102)
Marketable securities, estimated fair value	50,203	37,482
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	6,550	1,674
Cash equivalents, estimated fair value	6,550	1,674
Reverse repurchase agreements
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	6,250	7,250
Cash equivalents, estimated fair value	6,250	7,250
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost	32,236
Marketable securities, gross unrealized gains	2
Marketable securities, gross unrealized losses	(8)
Marketable securities, estimated fair value	32,230
U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost		1,502
Marketable securities, gross unrealized losses		(1)
Marketable securities, estimated fair value		1,501
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost	18,000	36,082
Marketable securities, gross unrealized losses	(27)	(101)
Marketable securities, estimated fair value	$ 17,973	$ 35,981

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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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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-Publisher FASB
-Name Accounting Standards Codification
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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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-Publisher FASB
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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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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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