Goodwill and Other Intangible Assets - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Sep. 30, 2023
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 499,235	$ 459,972
Accum. Amortization	400,523	383,666
Net	98,712	76,306
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	452,340	418,190
Accum. Amortization	373,860	358,855
Net	78,480	59,335
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	46,895	41,782
Accum. Amortization	26,663	24,811
Net	$ 20,232	$ 16,971