Stock-Based Compensation - Equity Incentive Plan Activity (Parenthetical) (Detail) - $ / shares shares in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock options outstanding include options vested and expected to vest in the future, weighted average remaining contractual life, in years	6 years 7 months 28 days
Unvested stock options	609,000	523,000
Unvested stock options, weighted average grant date fair value	$ 51.38	$ 52.95

Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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 (e)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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