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Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 43,386	$ 61,333
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(47)	(58)
Fair Value	43,339	61,275
Corporate Bonds (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,250	50,892
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36)	(48)
Fair Value	27,214	50,844
Commercial Paper (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,136	9,961
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(11)	(10)
Fair Value	$ 16,125	9,951
Certificates of Deposit (Due Within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		480
Gross Unrealized Gains		0
Fair Value		$ 480

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- Definition

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

+ References

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-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 investments in debt securities classified as available-for-sale.

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