Goodwill and Intangible Assets, net - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 437,821	$ 420,324
Accum. Amortization	334,381	330,031
Net	103,440	90,293
Customer Lists
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	399,588	382,942
Accum. Amortization	316,700	312,928
Net	82,888	70,014
Trade Names And Other Intangibles
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	38,233	37,382
Accum. Amortization	17,681	17,103
Net	$ 20,552	$ 20,279