Equity Incentive Plan - Additional Information (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Sep. 30, 2020 USD ($) shares / unit $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock Issuable per Restricted Stock Unit \| shares / unit	1
Stock-based compensation \| $	$ 115,149	$ 86,400	$ 82,939
Total unrecognized compensation cost \| $	$ 213,500
Weighted average remaining recognition period, in months	19 months
Common stock were available for grant under the 2000 plan \| shares	5,300
Common stock were reserved for issuance \| shares	3,500
ESPP maximum contribution percentage	10.00%
ESPP maximum contribution amount by employee \| $	$ 25,000
ESPP purchase price as a % of stock price	85.00%
Vesting period	1 year 3 months 18 days
Shares withheld for tax withholding obligation \| shares	500	500	700
Payments of withholding taxes in connection with vesting of stock-based awards \| $	$ 33,740	$ 44,366	$ 45,374
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $	$ 5,800	$ 6,200	$ 4,300
Restricted Shares and Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value per share \| $ / shares	$ 77.57	$ 82.77	$ 76.17
TSR Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value per share \| $ / shares	$ 106.69
Granted, shares \| shares	101
TSR Units \| Catch-Up Provision \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, shares \| shares	202

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ESPP purchase price as a % of stock price

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Common Stock Issuable per Restricted Stock Unit

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement.

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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.

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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).

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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).

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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.

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The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

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Number of shares used to settle grantee's tax withholding obligation for award under share-based payment arrangement.

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