Allowance for Loan and Lease Losses - Summary of Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	$ 70,774	$ 64,540	$ 63,503
Charge-offs	(44,750)	(37,061)	(19,904)
Recoveries	5,471	2,348	2,955
Provision	118,204	41,506	17,754
Allowance for loan losses, ending balance	149,887	70,774	64,540
Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	600
(Improvement) impairment of ASC 310-30 loans	188	(559)	232
Allowance for loan losses, ending balance	700	600
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	16,827	16,777	16,941
Charge-offs	(5,181)	(1,511)	(3,925)
Recoveries	1,395	567	533
Provision	71,474	1,514	3,231
Allowance for loan losses, ending balance	84,496	16,827	16,777
Commercial real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	(19)	(520)	(3)
Agriculture
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	30,819	28,121	25,757
Charge-offs	(21,705)	(24,847)	(9,473)
Recoveries	2,189	385	332
Provision	15,980	27,160	11,355
Allowance for loan losses, ending balance	27,018	30,819	28,121
Agriculture \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	(265)	0	150
Commercial non-real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	17,567	13,610	14,114
Charge-offs	(14,178)	(7,895)	(3,813)
Recoveries	1,018	392	994
Provision	23,192	11,431	2,315
Allowance for loan losses, ending balance	27,599	17,567	13,610
Commercial non-real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	0	29	0
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	4,095	4,749	5,347
Charge-offs	(615)	(998)	(569)
Recoveries	453	468	337
Provision	3,827	(56)	(451)
Allowance for loan losses, ending balance	8,202	4,095	4,749
Residential real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	442	(68)	85
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	427	257	329
Charge-offs	(87)	(452)	(192)
Recoveries	78	174	141
Provision	467	448	(21)
Allowance for loan losses, ending balance	915	427	257
Consumer \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	30	0	0
Other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, beginning balance	1,039	1,026	1,015
Charge-offs	(2,984)	(1,358)	(1,932)
Recoveries	338	362	618
Provision	3,264	1,009	1,325
Allowance for loan losses, ending balance	1,657	1,039	1,026
Other \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	$ 0	$ 0	$ 0

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount, after recoveries, of loans and leases that have been written-off.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount of expense related to estimated loss from loan and lease transactions.

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