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Stock-Based Compensation - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total compensation cost related to unvested stock options and RSU | $ 35,500,000 |  |
Weighted-average period of unvested stock options and RSU | 2 years 8 months 19 days |  |
Capitalized stock-based employee compensation expense | $ 0 | $ 0 | $ 0
Weighted-average estimated fair value per share of options granted (in dollars per share) | $ 5.25 | $ 6.37 | $ 11.39
Issuance of common stock under employee stock purchase plan (in shares) | 274,382 | 249,364 | 328,710
Stock issued under employee stock purchase plans, average exercise price (in dollars per share) | $ 7.34 | $ 8.53 | $ 4.51
Weighted-average estimated fair value of shares purchased under ESPP plan (in dollars per share) | $ 8.02 | $ 4.70 | $ 7.07
Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total compensation cost related to unvested stock options and RSU | $ 17,100,000 |  |
Weighted-average period of unvested stock options and RSU | 2 years 25 days |  |

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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + 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 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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