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Fair Value Measurements - Narrative (Details) - USD ($)
 6 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Sep. 30, 2024
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Available-for-sale securities purchased $ 809,552,000 $ 214,707,000  
Proceeds from sales of available-for-sale securities 797,000 176,402,000  
Held-to-maturity securities purchased 114,182,000 47,670,000  
Proceeds from sale of held-to-maturity securities $ 0 0  
Term of contractual due dates, mortgage-backed securities 25 years    
Accrued interest for available-for-sale securities $ 9,995,000   $ 9,311,000
Accrued interest for held-to-maturity securities 1,571,000   $ 1,154,000
Luther Burbank      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Proceeds from sales of available-for-sale securities   $ 171,000,000  
Consumer loans | Single-family residential      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Foreclosed residential real estate properties held as REO 0    
Real estate acquired through foreclosure $ 2,020,000