Loans - Troubled Debt Restructurings on Accruing and Nonaccrual Financing Receivables (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($) contract
Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 44,842	$ 19,783
Financing receivable, modifications, number of contracts | contract	22	7	26
Financing receivable, modifications, pre-modification recorded investment	$ 28,636	$ 12,867	$ 24,689
Financing receivable, modifications, post-modification recorded investment	$ 28,636	$ 12,867	$ 24,689
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	$ 30,073	$ 77,156
Financing receivable, modifications, number of contracts | contract	12	9	26
Financing receivable, modifications, pre-modification recorded investment	$ 10,383	$ 9,990	$ 24,139
Financing receivable, modifications, post-modification recorded investment	$ 10,383	$ 9,990	$ 24,139
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	$ 17,145	$ 2,649
Financing receivable, modifications, number of contracts | contract	2	1	2
Financing receivable, modifications, pre-modification recorded investment	$ 15,466	$ 2,041	$ 3,726
Financing receivable, modifications, post-modification recorded investment	15,466	2,041	$ 3,726
Commercial real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 904	$ 2,616
Financing receivable, modifications, number of contracts | contract	1	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 882	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	882	0	$ 0
Agriculture | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 22,929	$ 13,248
Financing receivable, modifications, number of contracts | contract	16	5	13
Financing receivable, modifications, pre-modification recorded investment	$ 11,537	$ 10,753	$ 18,902
Financing receivable, modifications, post-modification recorded investment	11,537	10,753	$ 18,902
Agriculture | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 24,762	$ 73,741
Financing receivable, modifications, number of contracts | contract	9	9	18
Financing receivable, modifications, pre-modification recorded investment	$ 5,802	$ 9,990	$ 20,197
Financing receivable, modifications, post-modification recorded investment	5,802	9,990	$ 20,197
Commercial non-real estate | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 4,398	$ 3,420
Financing receivable, modifications, number of contracts | contract	2	0	9
Financing receivable, modifications, pre-modification recorded investment	$ 1,445	$ 0	$ 2,044
Financing receivable, modifications, post-modification recorded investment	1,445	0	$ 2,044
Commercial non-real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 4,257	$ 656
Financing receivable, modifications, number of contracts | contract	2	0	3
Financing receivable, modifications, pre-modification recorded investment	$ 3,699	$ 0	$ 3,788
Financing receivable, modifications, post-modification recorded investment	3,699	0	$ 3,788
Residential real estate | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 263	$ 389
Financing receivable, modifications, number of contracts | contract	0	0	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 9
Financing receivable, modifications, post-modification recorded investment	0	0	$ 9
Residential real estate | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 102	$ 143
Financing receivable, modifications, number of contracts | contract	0	0	2
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 133
Financing receivable, modifications, post-modification recorded investment	0	0	$ 133
Consumer | Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 107	$ 77
Financing receivable, modifications, number of contracts | contract	2	1	1
Financing receivable, modifications, pre-modification recorded investment	$ 188	$ 73	$ 8
Financing receivable, modifications, post-modification recorded investment	188	73	$ 8
Consumer | Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 48	$ 0
Financing receivable, modifications, number of contracts | contract	0	0	3
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0	$ 21
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0	$ 21