Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	$ 3,142,763	$ 2,572,709
Held-to-maturity securities, at amortized cost	526,502	436,972
FHLB stock	133,964	95,617
Agency pass-through certificates
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities, at amortized cost	526,502	436,972
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	3,142,763	2,572,709
Held-to-maturity securities, at amortized cost	526,502	436,972
Carrying Amount \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,231,461	2,381,102
Carrying Amount \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB stock	133,964	95,617
Time deposits	9,573,442	9,556,785
Borrowings	2,763,758	3,267,589
Carrying Amount \| Level 2 \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	47,257	46,758
Other liabilities	47,816	47,388
Carrying Amount \| Level 2 \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	1,989	1,595
Carrying Amount \| Level 2 \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	6,876	0
Other liabilities	0	667
Carrying Amount \| Level 2 \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	120,666	117,271
Carrying Amount \| Level 2 \| Mortgage backed securities fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	8,682	0
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	273,755	314,204
Carrying Amount \| Level 2 \| Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	533,677	540,125
Carrying Amount \| Level 2 \| Municipal bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	35,349	35,073
Carrying Amount \| Level 2 \| Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	225,310	296,282
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	2,074,672	1,387,025
Held-to-maturity securities, at amortized cost	526,502	436,972
Carrying Amount \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	20,920,001	20,916,354
Junior subordinated debentures	51,180	50,718
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	3,142,763	2,572,709
Held-to-maturity securities, at amortized cost	484,592	401,046
Estimated Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,231,461	2,381,102
Estimated Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB stock	133,964	95,617
Time deposits	9,558,092	9,787,187
Borrowings	2,763,048	3,276,122
Estimated Fair Value \| Level 2 \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	47,257	46,758
Other liabilities	47,816	47,388
Estimated Fair Value \| Level 2 \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	1,989	1,595
Estimated Fair Value \| Level 2 \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	6,876	0
Other liabilities	0	667
Estimated Fair Value \| Level 2 \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	120,666	117,271
Estimated Fair Value \| Level 2 \| Mortgage backed securities fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	8,682	0
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	273,755	314,204
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	533,677	540,125
Estimated Fair Value \| Level 2 \| Municipal bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	35,349	35,073
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	225,310	296,282
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	2,074,672	1,387,025
Held-to-maturity securities, at amortized cost	484,592	401,046
Estimated Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	20,250,350	20,269,059
Junior subordinated debentures	$ 51,167	$ 50,240

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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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.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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