Employee Retirement Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Contribution Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible Enumeration]	Postretirement obligations
Benefit costs	$ 1,812	$ 1,403	$ 1,345
North America
Defined Contribution Plan Disclosure [Line Items]
Plan expense	$ 8,962	$ 7,658	$ 6,698