Goodwill and Other Intangible Assets - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 545,459	$ 499,235
Accum. Amortization	420,567	400,523
Net	124,892	98,712
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	493,431	452,340
Accum. Amortization	391,702	373,860
Net	101,729	78,480
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	52,028	46,895
Accum. Amortization	28,865	26,663
Net	$ 23,163	$ 20,232