Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 473,263	$ 459,972
Accum. Amortization	391,904	383,666
Net	81,359	76,306
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	431,540	418,190
Accum. Amortization	366,827	358,855
Net	64,713	59,335
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	41,723	41,782
Accum. Amortization	25,077	24,811
Net	$ 16,646	$ 16,971

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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-Topic 210
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 928
-SubTopic 340
-Name Accounting Standards Codification
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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 926
-SubTopic 20
-Name Accounting Standards Codification
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