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Stock-Based Compensation - Summary of ESPP Share Activity (Detail) - shares | 3 Months Ended | 9 Months Ended
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Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Purchases | (26,726) | (31,455) | (44,161) | (28,877) | (28,864) | (39,514) |
Available for future purchases, September 30, 2019 | 2,698,168 |  |  |  |  |  | 2,698,168
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Available for future purchases, January 1, 2019 |  |  | 183,244 |  |  |  | 183,244
Purchases |  |  |  |  |  |  | (102,343)
Available for future purchases, September 30, 2019 | 466,063 |  |  |  |  |  | 466,063
Employee Stock Purchase Plan [Member] | 2009 Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Additional shares reserved |  |  |  |  |  |  | 85,162
Employee Stock Purchase Plan [Member] | 2019 Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Additional shares reserved |  |  |  |  |  |  | 300,000

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- Definition Number of additional shares authorized for issuance under share-based payment arrangement. + References No definition available. + Details
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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 -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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