Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 446,551	$ 444,461
Accum. Amortization	353,774	348,987
Net	92,777	95,474
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	405,541	403,913
Accum. Amortization	333,555	329,406
Net	71,986	74,507
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	41,010	40,548
Accum. Amortization	20,219	19,581
Net	$ 20,791	$ 20,967