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Derivatives and Hedging Activities - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Sep. 30, 2020
--- | --- | ---
Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | $ 2,353,904 | $ 656,074
Derivative assets, fair value | 66,437 | 48,201
Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 744,822 | 2,849,390
Derivative liabilities, fair value | 9,714 | 90,129
Client swap program hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 653,904 | 656,074
Derivative assets, fair value | 5,104 | 48,201
Client swap program hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 653,904 | 656,074
Derivative liabilities, fair value | 5,104 | 48,201
Commercial loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 0
Derivative assets, fair value | 0 | 0
Commercial loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 90,918 | 93,316
Derivative liabilities, fair value | 4,610 | 8,492
Mortgage loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 500,000 | 0
Derivative assets, fair value | 2,135 | 0
Mortgage loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 500,000
Derivative liabilities, fair value | 0 | 16,061
Borrowings cash flow hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 1,200,000 | 0
Derivative assets, fair value | 59,198 | 0
Borrowings cash flow hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 1,600,000
Derivative liabilities, fair value | $ 0 | $ 17,375

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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959 + Details
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