Short-term Investments - Summary of Major Security and Type of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 87,122	$ 72,238
Gross Unrealized Gains	1
Gross Unrealized Losses	(21)	(46)
Fair Value	87,102	72,192
U.S. Government Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,992	11,894
Gross Unrealized Losses	(7)	(9)
Fair Value	11,985	11,885
Asset Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,484	8,389
Gross Unrealized Losses	(4)	(6)
Fair Value	9,480	8,383
Corporate Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,090	22,113
Gross Unrealized Losses	(10)	(31)
Fair Value	19,080	22,082
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	46,556	29,842
Gross Unrealized Gains	1
Fair Value	$ 46,557	$ 29,842

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

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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

No definition available.

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- Definition

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

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- Definition

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.

+ References

No definition available.

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