v3.26.1
Fair Value Measurements - Schedule of Fair Value of Financial Instruments by Balance Sheet Grouping (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Sep. 30, 2025
Financial assets    
Available-for-sale securities, at fair value $ 4,352,258 $ 3,533,201
Held-to-maturity securities, at amortized cost 745,727 645,802
FHLB stock 146,351 88,068
Agency pass-through certificates    
Financial assets    
Held-to-maturity securities, at amortized cost 745,727 645,802
Carrying Amount    
Financial assets    
Available-for-sale securities, at fair value 4,352,258 3,533,201
Held-to-maturity securities, at amortized cost 745,727 645,802
Estimated Fair Value    
Financial assets    
Available-for-sale securities, at fair value 4,352,258 3,533,201
Held-to-maturity securities, at amortized cost 712,117 612,739
Level 1 | Carrying Amount    
Financial assets    
Cash and cash equivalents 669,799 657,310
Level 1 | Estimated Fair Value    
Financial assets    
Cash and cash equivalents 669,799 657,310
Level 2 | Carrying Amount    
Financial assets    
FHLB stock 146,351 88,068
Financial liabilities    
Time deposits 8,377,230 9,131,104
Borrowings 3,062,441 1,765,604
Level 2 | Carrying Amount | Interest rate contracts    
Financial assets    
Other assets 34,461 37,347
Financial liabilities    
Other liabilities 34,527 37,818
Level 2 | Carrying Amount | Commercial loan fair value hedges    
Financial assets    
Other assets 0 1,611
Level 2 | Carrying Amount | Mortgage loan fair value hedges    
Financial assets    
Other assets 10,638 13,082
Financial liabilities    
Other liabilities 11,165 20,426
Level 2 | Carrying Amount | Mortgage backed securities fair value hedges    
Financial assets    
Other assets 294 0
Financial liabilities    
Other liabilities 8,468 15,086
Level 2 | Carrying Amount | Borrowings cash flow hedges    
Financial assets    
Other assets 97,378 99,231
Level 2 | Carrying Amount | U.S. government and agency securities    
Financial assets    
Available-for-sale securities, at fair value 202,727 235,919
Level 2 | Carrying Amount | Asset-backed securities    
Financial assets    
Available-for-sale securities, at fair value 474,365 506,334
Level 2 | Carrying Amount | Municipal bonds    
Financial assets    
Available-for-sale securities, at fair value 34,743 35,258
Level 2 | Carrying Amount | Corporate debt securities    
Financial assets    
Available-for-sale securities, at fair value 161,884 152,537
Level 2 | Carrying Amount | Agency pass-through certificates    
Financial assets    
Available-for-sale securities, at fair value 3,478,539 2,603,153
Held-to-maturity securities, at amortized cost 745,727 645,802
Level 2 | Estimated Fair Value    
Financial assets    
FHLB stock 146,351 88,068
Financial liabilities    
Time deposits 8,363,844 9,121,470
Borrowings 3,060,064 1,755,130
Level 2 | Estimated Fair Value | Interest rate contracts    
Financial assets    
Other assets 34,461 37,347
Financial liabilities    
Other liabilities 34,527 37,818
Level 2 | Estimated Fair Value | Commercial loan fair value hedges    
Financial assets    
Other assets 0 1,611
Level 2 | Estimated Fair Value | Mortgage loan fair value hedges    
Financial assets    
Other assets 10,638 13,082
Financial liabilities    
Other liabilities 11,165 20,426
Level 2 | Estimated Fair Value | Mortgage backed securities fair value hedges    
Financial assets    
Other assets 294 0
Financial liabilities    
Other liabilities 8,468 15,086
Level 2 | Estimated Fair Value | Borrowings cash flow hedges    
Financial assets    
Other assets 97,378 99,231
Level 2 | Estimated Fair Value | U.S. government and agency securities    
Financial assets    
Available-for-sale securities, at fair value 202,727 235,919
Level 2 | Estimated Fair Value | Asset-backed securities    
Financial assets    
Available-for-sale securities, at fair value 474,365 506,334
Level 2 | Estimated Fair Value | Municipal bonds    
Financial assets    
Available-for-sale securities, at fair value 34,743 35,258
Level 2 | Estimated Fair Value | Corporate debt securities    
Financial assets    
Available-for-sale securities, at fair value 161,884 152,537
Level 2 | Estimated Fair Value | Agency pass-through certificates    
Financial assets    
Available-for-sale securities, at fair value 3,478,539 2,603,153
Held-to-maturity securities, at amortized cost 712,117 612,739
Level 3 | Carrying Amount    
Financial assets    
Loans receivable 19,966,983 20,088,618
Financial liabilities    
Junior subordinated debentures 52,107 51,645
Level 3 | Estimated Fair Value    
Financial assets    
Loans receivable 19,636,861 19,681,909
Financial liabilities    
Junior subordinated debentures $ 52,093 $ 50,925