Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 536,713	$ 499,235
Accum. Amortization	410,135	400,523
Net	126,578	98,712
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	481,077	452,340
Accum. Amortization	382,372	373,860
Net	98,705	78,480
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	55,636	46,895
Accum. Amortization	27,763	26,663
Net	$ 27,873	$ 20,232