Short-term Investments - Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 69,681	$ 51,518
Gross unrealized gains	1	0
Gross unrealized losses	(41)	(99)
Fair value	69,641	51,419
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	47,299	35,567
Gross unrealized gains	1	0
Gross unrealized losses	(21)	(53)
Fair value	47,279	35,514
U.S. government-related debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	14,972	15,951
Gross unrealized gains	0	0
Gross unrealized losses	(11)	(46)
Fair value	14,961	$ 15,905
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	7,410
Gross unrealized gains	0
Gross unrealized losses	(9)
Fair value	$ 7,401

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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 investments in debt and equity securities classified as available-for-sale.

+ References

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

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