The Fair Value Option for Certain Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eligible item for the fair value option	$ 57,500		$ 57,500		$ 34,200
Loans greater than 90 days past due or in nonaccrual status	9,100		9,100		16,500
Unpaid principal balance greater than 90 days past due or in nonaccrual status	12,100		12,100		17,800
Net increase (decrease) in fair value of loans at fair value	35,541	$ 14,018	20,608	$ 33,234
Long-term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total unpaid principal balance of long-term loans	734,600		734,600		$ 778,800
Total change in fair value attributable to changes in specific credit risk	10,500	(400)	12,700	800
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	$ 35,500	$ 14,000	$ 20,600	$ 33,200