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Derivatives and Hedging Activities - Impact of Client Swap Program that are Not Designated in Accounting Hedges (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020
Client swap program hedges |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ 0 | $ 0 | $ 0 | $ 0
Other noninterest income | Pay fixed/receive floating swap |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | 40,929 | (28,184) | 53,305 | (22,776)
Other noninterest income | Receive fixed/pay floating swap |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ (40,929) | $ 28,184 | $ (53,305) | $ 22,776

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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4CC -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109998890-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
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