Loans - Troubled Debt Restructurings on Accruing and Nonaccrual Financing Receivables (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2020 USD ($) contract	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract
Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 35,205	$ 44,842
Financing receivable, modifications, number of contracts | contract	12	22	7
Financing receivable, modifications, pre-modification recorded investment	$ 24,982	$ 28,636	$ 12,867
Financing receivable, modifications, post-modification recorded investment	$ 24,982	$ 28,636	$ 12,867
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification	$ 0	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification	0	0	$ 0
Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 62,792	$ 30,073
Financing receivable, modifications, number of contracts | contract	18	12	9
Financing receivable, modifications, pre-modification recorded investment	$ 31,775	$ 10,383	$ 9,990
Financing receivable, modifications, post-modification recorded investment	$ 31,775	$ 10,383	$ 9,990
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification	$ 0	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification	0	0	$ 0
Commercial real estate | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 23,215	$ 17,145
Financing receivable, modifications, number of contracts | contract	4	2	1
Financing receivable, modifications, pre-modification recorded investment	$ 17,586	$ 15,466	$ 2,041
Financing receivable, modifications, post-modification recorded investment	17,586	15,466	$ 2,041
Commercial real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 11,913	$ 904
Financing receivable, modifications, number of contracts | contract	1	1	0
Financing receivable, modifications, pre-modification recorded investment	$ 2,216	$ 882	$ 0
Financing receivable, modifications, post-modification recorded investment	2,216	882	$ 0
Agriculture | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 2,976	$ 22,929
Financing receivable, modifications, number of contracts | contract	2	16	5
Financing receivable, modifications, pre-modification recorded investment	$ 993	$ 11,537	$ 10,753
Financing receivable, modifications, post-modification recorded investment	993	11,537	$ 10,753
Agriculture | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 45,971	$ 24,762
Financing receivable, modifications, number of contracts | contract	12	9	9
Financing receivable, modifications, pre-modification recorded investment	$ 27,807	$ 5,802	$ 9,990
Financing receivable, modifications, post-modification recorded investment	27,807	5,802	$ 9,990
Commercial non-real estate | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 8,734	$ 4,398
Financing receivable, modifications, number of contracts | contract	5	2	0
Financing receivable, modifications, pre-modification recorded investment	$ 6,353	$ 1,445	$ 0
Financing receivable, modifications, post-modification recorded investment	6,353	1,445	$ 0
Commercial non-real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 4,803	$ 4,257
Financing receivable, modifications, number of contracts | contract	5	2	0
Financing receivable, modifications, pre-modification recorded investment	$ 1,752	$ 3,699	$ 0
Financing receivable, modifications, post-modification recorded investment	1,752	3,699	$ 0
Residential real estate | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 277	$ 263
Financing receivable, modifications, number of contracts | contract	1	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 50	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	50	0	$ 0
Residential real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 74	$ 102
Financing receivable, modifications, number of contracts | contract	0	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	0	0	$ 0
Consumer | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 3	$ 107
Financing receivable, modifications, number of contracts | contract	0	2	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 188	$ 73
Financing receivable, modifications, post-modification recorded investment	0	188	$ 73
Consumer | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 31	$ 48
Financing receivable, modifications, number of contracts | contract	0	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0	$ 0