Short-term Investments - Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 58,267	$ 69,681
Gross unrealized gains	21	1
Gross unrealized losses	0	(41)
Fair value	58,288	69,641
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	39,914	47,299
Gross unrealized gains	15	1
Gross unrealized losses	0	(21)
Fair value	39,929	47,279
U.S. government-related debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	10,926	14,972
Gross unrealized gains	3	0
Gross unrealized losses	0	(11)
Fair value	10,929	14,961
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	7,427	7,410
Gross unrealized gains	3	0
Gross unrealized losses	0	(9)
Fair value	$ 7,430	$ 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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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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.

+ References

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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