Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Jun. 30, 2025	Sep. 30, 2024
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 545,459	$ 499,235
Accum. Amortization	415,368	400,523
Net	130,091	98,712
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	489,223	452,340
Accum. Amortization	387,104	373,860
Net	102,119	78,480
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	56,236	46,895
Accum. Amortization	28,264	26,663
Net	$ 27,972	$ 20,232