Cash Equivalents and Marketable Securities - Schedule of Estimated Fair Value of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	$ 7,620	$ 8,924
Cash equivalents, estimated fair value	7,620	8,924
Marketable securities, amortized cost	32,813	37,584
Marketable securities, gross unrealized losses	(98)	(102)
Marketable securities, estimated fair value	32,715	37,482
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
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	1,370	1,674
Cash equivalents, estimated fair value	1,370	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
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost	32,813	36,082
Marketable securities, gross unrealized losses	(98)	(101)
Marketable securities, estimated fair value	$ 32,715	$ 35,981

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
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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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