Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019
Loss Contingencies [Line Items]
Outstanding letters of credit		$ 18,600,000	$ 18,000,000.0
Data security incidents, remaining liability		$ 0	$ 0
Data security incidents
Loss Contingencies [Line Items]
Data security incident expenses	$ 7,900,000