v3.26.1
Fair Value Measurements - Schedule of Fair Value of Financial Instruments by Balance Sheet Grouping (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Sep. 30, 2025
Financial assets    
Available-for-sale securities, at fair value $ 4,190,263 $ 3,533,201
Held-to-maturity securities, at amortized cost 858,261 645,802
FHLB stock 155,392 88,068
Agency pass-through certificates    
Financial assets    
Held-to-maturity securities, at amortized cost 858,261 645,802
Carrying Amount    
Financial assets    
Available-for-sale securities, at fair value 4,190,263 3,533,201
Held-to-maturity securities, at amortized cost 858,261 645,802
Estimated Fair Value    
Financial assets    
Available-for-sale securities, at fair value 4,190,263 3,533,201
Held-to-maturity securities, at amortized cost 821,516 612,739
Level 1 | Carrying Amount    
Financial assets    
Cash and cash equivalents 676,467 657,310
Level 1 | Estimated Fair Value    
Financial assets    
Cash and cash equivalents 676,467 657,310
Level 2 | Carrying Amount    
Financial assets    
FHLB stock 155,392 88,068
Financial liabilities    
Time deposits 8,186,284 9,131,104
Borrowings 3,263,359 1,765,604
Level 2 | Carrying Amount | Interest rate contracts    
Financial assets    
Other assets 37,009 37,347
Financial liabilities    
Other liabilities 37,088 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 10,034 13,082
Financial liabilities    
Other liabilities 2,825 20,426
Level 2 | Carrying Amount | Mortgage backed securities fair value hedges    
Financial assets    
Other assets 1,947 0
Financial liabilities    
Other liabilities 3,728 15,086
Level 2 | Carrying Amount | Borrowings cash flow hedges    
Financial assets    
Other assets 104,446 99,231
Level 2 | Carrying Amount | U.S. government and agency securities    
Financial assets    
Available-for-sale securities, at fair value 190,897 235,919
Level 2 | Carrying Amount | Asset-backed securities    
Financial assets    
Available-for-sale securities, at fair value 426,124 506,334
Level 2 | Carrying Amount | Municipal bonds    
Financial assets    
Available-for-sale securities, at fair value 35,276 35,258
Level 2 | Carrying Amount | Corporate debt securities    
Financial assets    
Available-for-sale securities, at fair value 160,563 152,537
Level 2 | Carrying Amount | Agency pass-through certificates    
Financial assets    
Available-for-sale securities, at fair value 3,377,403 2,603,153
Held-to-maturity securities, at amortized cost 858,261 645,802
Level 2 | Estimated Fair Value    
Financial assets    
FHLB stock 155,392 88,068
Financial liabilities    
Time deposits 8,167,373 9,121,470
Borrowings 3,260,689 1,755,130
Level 2 | Estimated Fair Value | Interest rate contracts    
Financial assets    
Other assets 37,009 37,347
Financial liabilities    
Other liabilities 37,088 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 10,034 13,082
Financial liabilities    
Other liabilities 2,825 20,426
Level 2 | Estimated Fair Value | Mortgage backed securities fair value hedges    
Financial assets    
Other assets 1,947 0
Financial liabilities    
Other liabilities 3,728 15,086
Level 2 | Estimated Fair Value | Borrowings cash flow hedges    
Financial assets    
Other assets 104,446 99,231
Level 2 | Estimated Fair Value | U.S. government and agency securities    
Financial assets    
Available-for-sale securities, at fair value 190,897 235,919
Level 2 | Estimated Fair Value | Asset-backed securities    
Financial assets    
Available-for-sale securities, at fair value 426,124 506,334
Level 2 | Estimated Fair Value | Municipal bonds    
Financial assets    
Available-for-sale securities, at fair value 35,276 35,258
Level 2 | Estimated Fair Value | Corporate debt securities    
Financial assets    
Available-for-sale securities, at fair value 160,563 152,537
Level 2 | Estimated Fair Value | Agency pass-through certificates    
Financial assets    
Available-for-sale securities, at fair value 3,377,403 2,603,153
Held-to-maturity securities, at amortized cost 821,516 612,739
Level 3 | Carrying Amount    
Financial assets    
Loans receivable 20,017,876 20,088,618
Financial liabilities    
Junior subordinated debentures 52,338 51,645
Level 3 | Estimated Fair Value    
Financial assets    
Loans receivable 19,663,857 19,681,909
Financial liabilities    
Junior subordinated debentures $ 52,230 $ 50,925