The Fair Value Option for Certain Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ 35,400		$ 35,400		$ 34,200
Loans greater than 90 days past due or in nonaccrual status	36,200		36,200		16,500
Unpaid principal balance greater than 90 days past due or in nonaccrual status	62,600		62,600		17,800
Net increase (decrease) in fair value of loans at fair value	(22,118)	$ 16,429	(1,510)	$ 49,662
Long-term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total unpaid principal balance of long-term loans	699,900		699,900		$ 778,800
Total change in fair value attributable to changes in specific credit risk	23,300	4,800	35,900	5,600
Long-term loans | Noninterest income
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net increase (decrease) in fair value of loans at fair value	$ (22,100)	$ 16,400	$ (1,500)	$ 49,700