Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Matching contributions	$ 14,243	$ 14,533	$ 12,955
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Deferred compensation arrangement with individual, cash awards granted, percentage	6.00%
UNITED KINGDOM
Defined Benefit Plan Disclosure [Line Items]
Matching contributions	$ 1,021	$ 796	$ 707