FHLB Advances and Other Borrowings - Narrative (Details) - USD ($)	Sep. 30, 2020	Sep. 30, 2019
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 2,030,000,000.00	$ 1,800,000,000
Loans pledged to the Federal Home Loan Bank	4,070,000,000.00	4,200,000,000
Revolving line of credit \| FRB Discount Window Loan
Line of Credit Facility [Line Items]
Borrowing capacity	947,700,000	1,440,000,000
Loans pledged to the Federal Reserve Board Discount Window	1,170,000,000	1,720,000,000
FHLB \| Letters of credit
Line of Credit Facility [Line Items]
Borrowing capacity	75,000,000.0	170,000,000.0
FHLB \| Additional Letters of Credit
Line of Credit Facility [Line Items]
Borrowing capacity	$ 14,600,000	$ 14,900,000

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of other debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

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