Allowance for Loan and Lease Losses - Summary of Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	$ 72,781	$ 66,193	$ 70,774	$ 64,540
Charge-offs	(10,366)	(6,596)	(17,033)	(10,815)
Recoveries	1,740	733	2,311	1,390
Provision	71,699	7,406	79,750	13,006
Allowance for loan losses, ending balance	135,950	68,003	135,950	68,003
Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance			600
(Improvement) impairment of ASC 310-30 loans	96	267	148	(118)
Allowance for loan losses, ending balance	700		700
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	17,462	16,348	16,827	16,777
Charge-offs	(1,417)	(75)	(1,454)	(946)
Recoveries	114	162	234	259
Provision	48,285	(855)	48,857	(68)
Allowance for loan losses, ending balance	64,414	15,603	64,414	15,603
Commercial real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	(30)	23	(50)	(419)
Agriculture
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	32,029	31,785	30,819	28,121
Charge-offs	(4,522)	(5,767)	(9,128)	(7,028)
Recoveries	1,305	199	1,408	357
Provision	714	7,508	6,692	12,275
Allowance for loan losses, ending balance	29,526	33,725	29,526	33,725
Agriculture \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	0	0	(265)	0
Commercial non-real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	17,389	12,093	17,567	13,610
Charge-offs	(3,577)	(110)	(5,059)	(1,471)
Recoveries	59	104	172	228
Provision	17,895	962	19,086	682
Allowance for loan losses, ending balance	31,766	13,049	31,766	13,049
Commercial non-real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	0	0	0	0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	4,620	4,611	4,095	4,749
Charge-offs	(118)	(310)	(287)	(642)
Recoveries	147	125	312	287
Provision	3,602	(344)	3,794	(369)
Allowance for loan losses, ending balance	8,356	4,326	8,356	4,326
Residential real estate \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	105	244	442	301
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	288	430	427	257
Charge-offs	(25)	(85)	(45)	(334)
Recoveries	28	44	48	128
Provision	465	(15)	326	323
Allowance for loan losses, ending balance	777	374	777	374
Consumer \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	21	0	21	0
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, beginning balance	993	926	1,039	1,026
Charge-offs	(707)	(249)	(1,060)	(394)
Recoveries	87	99	137	131
Provision	738	150	995	163
Allowance for loan losses, ending balance	1,111	926	1,111	926
Other \| Receivables acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
(Improvement) impairment of ASC 310-30 loans	$ 0	$ 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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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