Allowance for Loan and Lease Losses - Summary of Allowance for Loan Losses by Type (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 42,993	$ 20,347
Collectively evaluated for impairment	106,200	49,862
ASC 310-30 loans	694	565
Total allowance	149,887	70,774	$ 64,540	$ 63,503
Financing Receivables
Individually evaluated for impairment	712,961	433,991
Collectively evaluated for impairment	7,810,598	8,263,022
ASC 310-30 loans	49,951	55,819
Loans Outstanding	8,573,510	8,752,832
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	25,087	4,159
Collectively evaluated for impairment	59,268	12,509
ASC 310-30 loans	141	159
Total allowance	84,496	16,827	16,777	16,941
Financing Receivables
Individually evaluated for impairment	232,501	54,275
Collectively evaluated for impairment	4,543,704	4,418,611
ASC 310-30 loans	20,928	22,124
Loans Outstanding	4,797,133	4,495,010
Agriculture
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	8,151	8,234
Collectively evaluated for impairment	18,867	22,320
ASC 310-30 loans	0	265
Total allowance	27,018	30,819	28,121	25,757
Financing Receivables
Individually evaluated for impairment	361,786	329,479
Collectively evaluated for impairment	1,188,442	1,501,164
ASC 310-30 loans	2,969	2,756
Loans Outstanding	1,553,197	1,833,399
Commercial non-real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	7,822	6,062
Collectively evaluated for impairment	19,748	11,476
ASC 310-30 loans	29	29
Total allowance	27,599	17,567	13,610	14,114
Financing Receivables
Individually evaluated for impairment	106,363	42,910
Collectively evaluated for impairment	1,200,794	1,480,949
ASC 310-30 loans	192	221
Loans Outstanding	1,307,349	1,524,080
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,903	1,795
Collectively evaluated for impairment	5,805	2,188
ASC 310-30 loans	494	112
Total allowance	8,202	4,095	4,749	5,347
Financing Receivables
Individually evaluated for impairment	12,213	7,119
Collectively evaluated for impairment	777,610	763,645
ASC 310-30 loans	25,570	30,280
Loans Outstanding	815,393	801,044
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	30	97
Collectively evaluated for impairment	855	330
ASC 310-30 loans	30	0
Total allowance	915	427	257	329
Financing Receivables
Individually evaluated for impairment	98	208
Collectively evaluated for impairment	62,701	51,112
ASC 310-30 loans	292	438
Loans Outstanding	63,091	51,758
Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	1,657	1,039
ASC 310-30 loans	0	0
Total allowance	1,657	1,039	$ 1,026	$ 1,015
Financing Receivables
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	37,347	47,541
ASC 310-30 loans	0	0
Loans Outstanding	$ 37,347	$ 47,541

Financing Receivable, Additions With Deteriorated Credit Quality

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Financing Receivable, Allowance For Credit Losses, Additions With Deteriorated Credit Quality

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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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, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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