Marketable Securities - Fair Value of Available for Sale Marketable Securities (Detail) - Level 2 [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Adjusted Cost	$ 36,313	$ 34,520
Available for sale securities, Unrealized Gains	12
Available for sale securities, Unrealized Losses	(3)	(23)
Available for sale securities, Fair Value	36,322	34,497
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Adjusted Cost	27,235	31,521
Available for sale securities, Unrealized Gains	11
Available for sale securities, Unrealized Losses	(3)	(23)
Available for sale securities, Fair Value	27,243	31,498
Government-Sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Adjusted Cost	9,078	2,999
Available for sale securities, Unrealized Gains	1
Available for sale securities, Fair Value	$ 9,079	$ 2,999

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)
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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

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-Publisher FASB
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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)
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