Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 83,251	$ 46,181
Gross Unrealized Gains		12
Gross Unrealized Losses	(70)	(9)
Estimated Fair Value	83,181	46,184
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,988	8,268
Gross Unrealized Gains		8
Gross Unrealized Losses	(37)
Estimated Fair Value	39,951	8,276
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,799	3,501
Gross Unrealized Gains		1
Gross Unrealized Losses	(5)
Estimated Fair Value	7,794	3,502
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,919	16,683
Gross Unrealized Losses	(12)	(6)
Estimated Fair Value	15,907	16,677
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,545	15,206
Gross Unrealized Gains		3
Gross Unrealized Losses	(16)	(2)
Estimated Fair Value	$ 19,529	15,207
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,523
Gross Unrealized Losses		(1)
Estimated Fair Value		$ 2,522

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

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

+ References

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This item represents the cost of debt 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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