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Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period, minimum | 3 years |  |
Number of shares of common stock that may be issued (in shares) | 1,000,000 |  |
Restricted Stock Awards [Abstract] |  |  |
Dividend yield percent | 0.00% | 0.00% | 0.00%
Stock Option Awards [Abstract] |  |  |
Total unrecognized compensation costs | $ 10.8 |  |
Weighted average period for recognition of unrecognized compensation costs | 1 year 9 months |  |
Restricted Stock Awards |  |  |
Restricted Stock Awards [Abstract] |  |  |
Vested restricted stock withheld for tax withholding obligations (in shares) | 41,353 | 28,731 | 34,922
Vested restricted stock withheld for tax withholding obligations, weighted average price (in dollars per share) | $ 77.07 | $ 79.23 | $ 81.65
Fair value, restricted stock | $ 9.8 | $ 7.6 | $ 10.5
Actual tax benefit realized | $ 3.7 | $ 2.9 | $ 4.0
Restricted Stock Awards | Performance Vesting |  |  |
Restricted Stock Awards [Abstract] |  |  |
Granted (in shares) | 91,662 |  |
Granted, weighted average grant date fair value (in dollars per share) | $ 76.19 |  |
Expiration date | Sep. 30, 2023 |  |
Restricted Stock Awards | Time Vesting |  |  |
Restricted Stock Awards [Abstract] |  |  |
Granted (in shares) | 33,350 |  |
Granted, weighted average grant date fair value (in dollars per share) | $ 76.13 |  |
Restricted Stock Awards | Executive Officers and Key Employees | Performance Vesting |  |  |
Restricted Stock Awards [Abstract] |  |  |
Granted, weighted average grant date fair value (in dollars per share) |  | $ 80.72 | $ 79.88
Maximum amount of shares pursuant to restricted stock awards | 200.00% |  |
Number of shares issued for maximum performance payout (in shares) | 519,888 |  |
Number of shares elected to be defer at target payout (in shares) | 59,408 | 80,760 |
Restricted Stock Awards | Executive Officers and Key Employees | Time Vesting |  |  |
Restricted Stock Awards [Abstract] |  |  |
Granted (in shares) | 33,350 |  |
Granted, weighted average grant date fair value (in dollars per share) | $ 76.13 | $ 76.66 | 76.60
Restricted Stock Awards | Non Employee Directors | Time Vesting |  |  |
Restricted Stock Awards [Abstract] |  |  |
Granted (in shares) | 10,560 |  |
Granted, weighted average grant date fair value (in dollars per share) | $ 84.58 | $ 78.69 | $ 63.20

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- Definition Share based compensation arrangement by share based payment award number of shares elected to be defer at target payout. + References No definition available. + Details
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- Definition Amount of cost not yet recognized for nonvested award under share-based payment arrangement. + 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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- 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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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Date the equity-based award expires, in CCYY-MM-DD format. + 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 (a)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + 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 (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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