Financial Instruments - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Cost	$ 48,686	$ 103,148
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(456)	(303)
Estimated Fair Values	48,230	102,845
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Cost	33,905	45,803
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(377)	(230)
Estimated Fair Values	33,528	45,573
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost		2,392
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Estimated Fair Values		2,390
Treasury and agency notes and bills
Debt Securities, Available-for-sale [Line Items]
Cost	6,000	6,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(79)	(71)
Estimated Fair Values	5,921	5,929
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Cost		8,540
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Values		8,540
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cost	8,781	40,413
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Values	8,781	40,413
Cash and cash equivalents
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Values	8,781	40,413
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Values	$ 39,449	$ 62,432

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).

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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+ References

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-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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