Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 447,914	$ 444,461
Accum. Amortization	358,313	348,987
Net	89,601	95,474
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	406,614	403,913
Accum. Amortization	337,508	329,406
Net	69,106	74,507
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	41,300	40,548
Accum. Amortization	20,805	19,581
Net	$ 20,495	$ 20,967