Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 441,607	$ 420,324
Accum. Amortization	339,128	330,031
Net	102,479	90,293
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	401,630	382,942
Accum. Amortization	320,809	312,928
Net	80,821	70,014
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	39,977	37,382
Accum. Amortization	18,319	17,103
Net	$ 21,658	$ 20,279