Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	$ 2,743,731	$ 2,572,709
Held-to-maturity securities, at amortized cost	537,348	436,972
FHLB stock	128,396	95,617
Agency pass-through certificates
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities, at amortized cost	537,348	436,972
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	2,743,731	2,572,709
Held-to-maturity securities, at amortized cost	537,348	436,972
Carrying Amount \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,507,735	2,381,102
Carrying Amount \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB stock	128,396	95,617
Time deposits	9,584,918	9,556,785
Borrowings	2,863,675	3,267,589
Carrying Amount \| Level 2 \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	56,862	46,758
Other liabilities - client swap program hedges	57,486	47,388
Carrying Amount \| Level 2 \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	2,423	1,595
Carrying Amount \| Level 2 \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	33,103	0
Other liabilities - client swap program hedges	0	667
Carrying Amount \| Level 2 \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	138,870	117,271
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	294,193	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	546,782	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	34,793	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	267,874	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	1,600,089	1,387,025
Held-to-maturity securities, at amortized cost	537,348	436,972
Carrying Amount \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	21,060,501	20,916,354
Junior subordinated debentures	50,952	50,718
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities, at fair value	2,743,731	2,572,709
Held-to-maturity securities, at amortized cost	485,106	401,046
Estimated Fair Value \| Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1,507,735	2,381,102
Estimated Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB stock	128,396	95,617
Time deposits	9,575,502	9,787,187
Borrowings	2,870,221	3,276,122
Estimated Fair Value \| Level 2 \| Client swap program hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	56,862	46,758
Other liabilities - client swap program hedges	57,486	47,388
Estimated Fair Value \| Level 2 \| Commercial loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	2,423	1,595
Estimated Fair Value \| Level 2 \| Mortgage loan fair value hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	33,103	0
Other liabilities - client swap program hedges	0	667
Estimated Fair Value \| Level 2 \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	138,870	117,271
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	294,193	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	546,782	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	34,793	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	267,874	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	1,600,089	1,387,025
Held-to-maturity securities, at amortized cost	485,106	401,046
Estimated Fair Value \| Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	20,295,920	20,269,059
Junior subordinated debentures	$ 50,834	$ 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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