The Fair Value Option For Certain Loans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ 37,300	$ 34,200
Loans greater than 90 days past due or in nonaccrual status	21,700	16,500
Loans greater than 90 days past due or in nonaccrual status, unpaid principal balance	26,200	17,800
Net (decrease) increase in fair value of loans at fair value	(32,529)	61,412	$ (45,407)
Loans Receivable and Written Loan Commitments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	59,400	7,700	200
Fair value, option, aggregate differences, long-term debt instruments	617,900	778,800
Loans Receivable and Written Loan Commitments | Noninterest income
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net (decrease) increase in fair value of loans at fair value	$ (32,500)	$ 61,400	$ (45,400)