Intangibles, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 421,241	$ 420,112
Accum. Amortization	322,996	312,424
Net	98,245	107,688
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	383,388	382,373
Accum. Amortization	306,657	297,221
Net	76,731	85,152
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	37,853	37,739
Accum. Amortization	16,339	15,203
Net	$ 21,514	$ 22,536