Stock-Based Compensation - Summary of ESPP Share Activity (Detail) - 2009 Employee Stock Purchase Plan [Member]	9 Months Ended
Sep. 30, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Available for future purchases, January 1, 2018	225,418
Additional shares reserved under 2009 ESPP	83,838
Purchases	(97,257)
Available for future purchases, September 30, 2018	211,999

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.

No definition available.

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- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

+ References

No definition available.

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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-Name Accounting Standards Codification
-Topic 505
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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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