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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Mar. 18, 2021 | Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of options outstanding | 3,576,575 |  | 3,576,575 |  |  | 4,539,403
RSU awards vesting, description |  |  | At vesting, each outstanding RSU will be exchanged for one share of the Company’s Common Stock. RSU awards generally vest subject to the satisfaction of service requirements or the satisfaction of both service requirements and achievement of certain performance targets. |  |  |
Employee Stock Option |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation expense | $ 3.2 | $ 2.7 | $ 9.6 | $ 6.8 |  |
Grant date fair value of the options granted | 0.9 | 3.4 | 9.0 | 30.0 |  |
Intrinsic value of options exercised | 10.2 | 9.5 | 63.0 | 39.8 |  |
Unrecognized pre-tax compensation expense | 31.3 |  | $ 31.3 |  |  |
Weighted average period to recognize pre-tax compensation expense |  |  | 2 years 8 months 12 days |  |  |
Dividend yield |  |  | 0.00% |  |  |
Restricted Stock Units (RSUs) |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation expense | 15.4 | 7.4 | $ 32.5 | 20.7 |  |
Weighted average period to recognize pre-tax compensation expense |  |  | 2 years 8 months 12 days |  |  |
Grant date fair value of other than options granted | 3.1 | $ 5.9 | $ 112.1 | $ 141.8 |  |
Unrecognized pre-tax compensation expense | $ 123.5 |  | 123.5 |  |  |
Additional unrecognized pre-tax compensation expense |  |  | $ 70.1 |  |  |
2004 Equity Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares reserve for issuance | 429,141 |  | 429,141 |  |  |
2004 Equity Incentive Plan | Employee Stock Option |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of options outstanding | 429,141 |  | 429,141 |  |  |
2013 Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares reserve for issuance | 5,293,368 |  | 5,293,368 |  |  |
Number of shares forfeited |  |  | 81,939 |  |  |
2013 Incentive Plan | Employee Stock Option |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of options outstanding | 2,127,918 |  | 2,127,918 |  |  |
2013 Incentive Plan | Restricted Stock Units (RSUs) |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of restricted stock units outstanding | 3,165,450 |  | 3,165,450 |  |  |
Outside Of Equity Compensation Plans |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares reserve for issuance | 1,001,516 |  | 1,001,516 |  |  |
Outside Of Equity Compensation Plans | Restricted Stock Units (RSUs) |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Shares reserve for issuance | 636,700 |  | 636,700 |  |  |
2021 Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of shares authorized | 8,020,439 |  | 8,020,439 |  | 8,000,000 |
2021 Incentive Plan | Employee Stock Option |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of options outstanding | 18,000 |  | 18,000 |  |  |
2021 Incentive Plan | Restricted Stock Units (RSUs) |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Number of restricted stock units outstanding | 43,500 |  | 43,500 |  |  |

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