Summary of Significant Accounting Policies - Notes Receivable (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on restructured notes receivable					$ 3.0
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured notes receivable, estimated settlement amount					14.0
Not primary beneficiary, variable interest
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on restructured notes receivable		$ 3.0
Restructured notes receivable, estimated settlement amount					$ 14.0
Principal collections on notes receivable	$ 6.0		$ 34.1	$ 32.4
Fee Due Date Two and Three | Not primary beneficiary, variable interest
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured notes receivable, estimated settlement amount	$ 8.0		$ 8.0