Loans - Summary of Troubled Debt Restructurings on Accruing and Nonaccrual Financing Receivables (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020 USD ($) contract	Jun. 30, 2019 USD ($) contract	Jun. 30, 2020 USD ($) contract	Jun. 30, 2019 USD ($) contract	Sep. 30, 2019 USD ($)
Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 43,550		$ 43,550		$ 44,842
Financing receivable, modifications, number of contracts | contract	1	18	9	19
Financing receivable, modifications, pre-modification recorded investment	$ 12,001	$ 27,003	$ 21,019	$ 27,092
Financing receivable, modifications, post-modification recorded investment	$ 12,001	$ 27,003	$ 21,019	$ 27,092
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification recorded investment	0	$ 0	0	$ 0
Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 55,183		$ 55,183		30,073
Financing receivable, modifications, number of contracts | contract	4	12	17	12
Financing receivable, modifications, pre-modification recorded investment	$ 20,264	$ 10,383	$ 24,765	$ 10,383
Financing receivable, modifications, post-modification recorded investment	$ 20,264	$ 10,383	$ 24,765	$ 10,383
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification recorded investment	0	$ 0	0	$ 0
Commercial real estate | Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 31,627		$ 31,627		17,145
Financing receivable, modifications, number of contracts | contract	1	2	2	2
Financing receivable, modifications, pre-modification recorded investment	$ 12,001	$ 15,466	$ 14,880	$ 15,466
Financing receivable, modifications, post-modification recorded investment	12,001	$ 15,466	14,880	$ 15,466
Commercial real estate | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 1,762		$ 1,762		904
Financing receivable, modifications, number of contracts | contract	0	1	1	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 882	$ 2,216	$ 882
Financing receivable, modifications, post-modification recorded investment	0	$ 882	2,216	$ 882
Agriculture | Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 2,989		$ 2,989		22,929
Financing receivable, modifications, number of contracts | contract	0	16	2	16
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 11,537	$ 993	$ 11,537
Financing receivable, modifications, post-modification recorded investment	0	$ 11,537	993	$ 11,537
Agriculture | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 47,791		$ 47,791		24,762
Financing receivable, modifications, number of contracts | contract	1	9	11	9
Financing receivable, modifications, pre-modification recorded investment	$ 19,342	$ 5,802	$ 20,797	$ 5,802
Financing receivable, modifications, post-modification recorded investment	19,342	$ 5,802	20,797	$ 5,802
Commercial non-real estate | Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 8,645		$ 8,645		4,398
Financing receivable, modifications, number of contracts | contract	0	0	4	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 5,096	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0	5,096	$ 0
Commercial non-real estate | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 5,508		$ 5,508		4,257
Financing receivable, modifications, number of contracts | contract	3	2	5	2
Financing receivable, modifications, pre-modification recorded investment	$ 922	$ 3,699	$ 1,752	$ 3,699
Financing receivable, modifications, post-modification recorded investment	922	$ 3,699	1,752	$ 3,699
Residential real estate | Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 285		$ 285		263
Financing receivable, modifications, number of contracts | contract	0	0	1	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 50	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0	50	$ 0
Residential real estate | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 86		$ 86		102
Financing receivable, modifications, number of contracts | contract	0	0	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0	0	$ 0
Consumer | Accruing
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 4		$ 4		107
Financing receivable, modifications, number of contracts | contract	0	0	0	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 89
Financing receivable, modifications, post-modification recorded investment	0	$ 0	0	$ 89
Consumer | Nonaccrual
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded value of TDR balance	$ 36		$ 36		$ 48
Financing receivable, modifications, number of contracts | contract	0	0	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0	$ 0	$ 0