Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 183,197	$ 132,132
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(189)	(119)
Available-for-sale Securities	183,008	132,013
Money Market Fund [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,152	29,848
Gross Unrealized Gains	0	0
Available-for-sale Securities	21,152	29,848
U.S. Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,919	29,005
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(31)	(52)
Available-for-sale Securities	22,888	28,953
Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	39,690	46,184
Gross Unrealized Gains	0	0
Available-for-sale Securities	39,690	46,184
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,037
Gross Unrealized Gains	0
Gross Unrealized Losses	(8)
Available-for-sale Securities	29,029
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	70,399	27,095
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(150)	(67)
Available-for-sale Securities	$ 70,249	$ 27,028

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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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).

+ 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 (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

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

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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