Marketable Securities - Summary of Available-for-Sale Short-Term and Long-Term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 137,219	$ 36,230
Gross Unrealized Gains	46	2
Aggregate Fair Value	137,265	36,232
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,997
Gross Unrealized Gains	7
Aggregate Fair Value	2,004
Corporate Debt Securities [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,219	36,230
Gross Unrealized Gains	43	2
Aggregate Fair Value	134,262	$ 36,232
Corporate Debt Securities [Member] \| Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,997
Gross Unrealized Gains	7
Aggregate Fair Value	2,004
U.S. Goverment and Goverment Agency Debt Securities [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000
Gross Unrealized Gains	3
Aggregate Fair Value	$ 3,003

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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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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