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Allowance for Losses on Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit losses	$ 2,750	$ 16,000	$ 2,750	$ 16,000
Allowance for credit loss, charge-off (recovery)	5,063	$ 146	5,294	$ 33
Gross loans	$ 21,122,710		$ 21,122,710		$ 21,120,107
Percent past due	0.27%		0.27%		0.25%
Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	$ 59,886		$ 59,886		$ 69,541
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 70,884		$ 70,884		$ 77,418
Ratio of non-performing assets to total assets	0.26%		0.26%		0.28%
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 12,238,959		$ 12,238,959		$ 12,205,420
Percent past due	0.16%		0.16%		0.16%
Commercial loans | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	$ 39,797		$ 39,797		$ 46,299
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,384,745		$ 2,384,745		$ 2,332,732
Percent past due	0.00%		0.00%		0.09%
Commercial loans | Commercial & industrial | Financial Asset Originated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual	$ 0		$ 0		$ 0

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

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Amount, excluding accrued interest, of decrease (increase) in allowance for credit loss on financing receivable from writeoff (recovery). Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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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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-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

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
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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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
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