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Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses	$ 16,000	$ 3,500	$ 16,000	$ 6,000
Charge-offs, net of recoveries	146	$ 5,877	33	$ 5,388
Gross loans	$ 20,996,836		$ 20,996,836		$ 17,653,757
Percent past due	0.36%		0.36%		0.36%
Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 60,806		$ 60,806		$ 50,422
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 68,361		$ 68,361		$ 57,924
Ratio of non-performing assets to total assets	0.23%		0.23%		0.26%

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Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Section 50
-Paragraph 5
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-Publisher FASB
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Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
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Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Name Accounting Standards Codification
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Ratio of Non-Performing Assets to Total Assets

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