Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 462,657	$ 459,972
Accum. Amortization	388,688	383,666
Net	73,969	76,306
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	420,460	418,190
Accum. Amortization	362,914	358,855
Net	57,546	59,335
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	42,197	41,782
Accum. Amortization	25,774	24,811
Net	$ 16,423	$ 16,971