Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,185	$ 114,040
Gross Unrealized Loss	(8)	(145)
Fair Value	17,177	113,895
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,397	6,198
Fair Value	1,397	6,198
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,298	58,937
Gross Unrealized Loss	(2)	(72)
Fair Value	4,296	58,865
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,490	16,992
Gross Unrealized Loss	(6)	(30)
Fair Value	$ 11,484	16,962
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		8,916
Fair Value		8,916
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,997
Gross Unrealized Loss		(43)
Fair Value		$ 22,954

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).

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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- Definition

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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- Definition

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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-Name Accounting Standards Codification
-Topic 320
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-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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