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Stock-Based Compensation - Narrative (Detail) - USD ($) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Apr. 05, 2019 | Apr. 11, 2012 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2012 | May 09, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock based compensation cost |  |  | $ 21,150,000 | $ 19,180,000 | $ 57,554,000 | $ 54,835,000 |  |  |
Options granted, contractual period, years |  |  |  |  | 10 years |  |  |  |
Options granted (in shares) |  |  |  |  | 1,831,180 | 431,924 |  |  |
Unrecognized stock-based compensation cost for options |  |  | 26,843,000 |  | $ 26,843,000 |  |  |  |
Restrcited Stock Unit vesting right (in shares) |  |  |  |  | 1 |  |  |  |
Restrcited Stock Unit vesting right, fair value (in shares) |  |  |  |  | 1 |  |  |  |
Performance Units vesting right (in shares) |  |  |  |  | 1 |  |  |  |
Performance Units vesting right, fair value (in shares) |  |  |  |  | 1 |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 3 years |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 5 years |  |  |  |
Employee Stock Option |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Weighted average remaining requisite vesting period |  |  |  |  | 3 years 3 months 18 days |  |  |  |
Restricted Share Units (RSUs) |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Weighted average remaining requisite vesting period |  |  |  |  | 2 years 4 months 24 days |  |  |  |
Shares to be issued on vested awards other than options (in shares) |  |  |  |  |  |  | 34,092 |  |
Unrecognized stock-based compensation cost |  |  | 21,417,000 |  | $ 21,417,000 |  |  |  |
Restricted Share Units (RSUs) | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 3 months |  |  |  |
Restricted Share Units (RSUs) | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 4 years |  |  |  |
Performance Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Weighted average remaining requisite vesting period |  |  |  |  | 1 year 10 months 24 days |  |  |  |
Unrecognized stock-based compensation cost |  |  | $ 65,843,000 |  | $ 65,843,000 |  |  |  |
Performance Units | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 6 months |  |  |  |
Performance Units | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award, vesting period |  |  |  |  | 3 years |  |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Fair value per share allowed to eligible employees to purchase through payroll deductions (in percentage) |  |  |  |  | 90.00% |  |  |  |
Maximum percentage of employee's base salary allowed to be purchased (in percentage) |  |  | 15.00% |  | 15.00% |  |  |  |
Maximum dollar amount of common shares allowed to be purchased |  |  |  |  | $ 25,000 |  |  |  |
Common shares reserved for issuance (in shares) |  |  | 4,200,000 |  | 4,200,000 |  |  |  |
Issuance of common shares under the employee stock purchase plan (in shares) |  |  |  |  | 216,378 | 241,953 |  |  |
Compensation expense for ESPP |  |  | $ 335,000 | $ 307,000 | $ 1,050,000 | $ 983,000 |  |  |
2007 Omnibus Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Amended Omnibus Plan, increase in number of common shares authorized for issuance (in shares) | 8,000,000 | 5,593,200 |  |  |  |  |  | 8,858,823 |
Number of common shares authorized for issuance (in shares) | 23,000,000 | 15,000,000 |  |  |  |  |  |  |
2017 Omnibus Incentive Compensation Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of common shares authorized for issuance (in shares) |  |  |  |  |  |  |  |  | 15,000,000

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