Goodwill and Other Intangible Assets - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 444,461	$ 420,324
Accum. Amortization	348,987	330,031
Net	95,474	90,293
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	403,913	382,942
Accum. Amortization	329,406	312,928
Net	74,507	70,014
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	40,548	37,382
Accum. Amortization	19,581	17,103
Net	$ 20,967	$ 20,279