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Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Sep. 30, 2023
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | $ 2,428,769 | $ 1,995,097
Held-to-maturity securities, at amortized cost | 447,638 | 423,586
Loans held for sale | 468,527 | 0
FHLB stock | 107,282 | 126,820
Agency pass-through certificates |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities, at amortized cost | 447,638 | 423,586
Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 2,428,769 | 1,995,097
Held-to-maturity securities, at amortized cost | 447,638 | 423,586
Carrying Amount | Level 1 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 2,492,504 | 980,649
Carrying Amount | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans held for sale | 468,527 | 0
FHLB stock | 107,282 | 126,820
Time deposits | 9,255,760 | 5,305,016
Carrying Amount | Level 2 | Borrowings |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | 3,934,514 | 3,650,000
Carrying Amount | Level 2 | Senior Debt |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | 94,361 | 0
Carrying Amount | Level 2 | Client swap program hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 63,374 | 78,797
Other liabilities - client swap program hedges | 64,075 | 79,668
Carrying Amount | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 2,534 | 3,405
Carrying Amount | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 59,243 | 46,396
Carrying Amount | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 151,333 | 184,373
Carrying Amount | Level 2 | U.S. government and agency securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 292,775 | 217,053
Carrying Amount | Level 2 | Asset-backed securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 507,812 | 588,016
Carrying Amount | Level 2 | Municipal bonds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 35,017 | 34,662
Carrying Amount | Level 2 | Corporate debt securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 241,167 | 242,522
Carrying Amount | Level 2 | Agency pass-through certificates |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 1,351,998 | 912,844
Held-to-maturity securities, at amortized cost | 447,638 | 423,586
Carrying Amount | Level 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 20,873,919 | 17,476,550
Carrying Amount | Level 3 | Junior subordinated deferrable interest debentures |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | 50,485 | 0
Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 2,428,769 | 1,995,097
Held-to-maturity securities, at amortized cost | 398,005 | 355,188
Estimated Fair Value | Level 1 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 2,492,504 | 980,649
Estimated Fair Value | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans held for sale | 468,527 | 0
FHLB stock | 107,282 | 126,820
Time deposits | 9,188,683 | 5,232,689
Estimated Fair Value | Level 2 | Borrowings |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | 3,929,935 | 3,653,229
Estimated Fair Value | Level 2 | Senior Debt |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | 94,752 | 0
Estimated Fair Value | Level 2 | Client swap program hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 63,374 | 78,797
Other liabilities - client swap program hedges | 64,075 | 79,668
Estimated Fair Value | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 2,534 | 3,405
Estimated Fair Value | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 59,243 | 46,396
Estimated Fair Value | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 151,333 | 184,373
Estimated Fair Value | Level 2 | U.S. government and agency securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 292,775 | 217,053
Estimated Fair Value | Level 2 | Asset-backed securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 507,812 | 588,016
Estimated Fair Value | Level 2 | Municipal bonds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 35,017 | 34,662
Estimated Fair Value | Level 2 | Corporate debt securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 241,167 | 242,522
Estimated Fair Value | Level 2 | Agency pass-through certificates |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 1,351,998 | 912,844
Held-to-maturity securities, at amortized cost | 398,005 | 355,188
Estimated Fair Value | Level 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 20,038,322 | 16,559,758
Estimated Fair Value | Level 3 | Junior subordinated deferrable interest debentures |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Debt | $ 50,535 | $ 0

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- Definition Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Fair value, after the 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. 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