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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Mar. 31, 2025
USD ($)
security
Sep. 30, 2024
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 205 209
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (1,586) $ (429)
12 months or more, unrealized gross losses (63,661) (54,270)
Total unrealized gross losses (65,247) (54,699)
Less than 12 months, fair value 253,669 239,441
12 months or more, fair value 952,292 1,125,962
Total, fair value 1,205,961 1,365,403
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (9) 0
Less than 12 months, fair value 34,152 0
12 months or more, unrealized gross losses (42,883) (36,839)
12 months or more, fair value 324,360 348,573
Total unrealized gross losses (42,892) (36,839)
Total, fair value 358,512 348,573
Less than 12 months, unrealized gross losses (1,595) (429)
Less than 12 months, fair value 287,821 239,441
12 months or more, unrealized gross losses (106,544) (91,109)
12 months or more, fair value 1,276,652 1,474,535
Total unrealized gross losses (108,139) (91,538)
Total fair value 1,564,473 1,713,976
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (229) 0
12 months or more, unrealized gross losses (11,834) (10,993)
Total unrealized gross losses (12,063) (10,993)
Less than 12 months, fair value 19,771 0
12 months or more, fair value 119,597 146,060
Total, fair value 139,368 146,060
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 (15)
12 months or more, unrealized gross losses (669) (394)
Total unrealized gross losses (669) (409)
Less than 12 months, fair value 0 19,985
12 months or more, fair value 14,779 15,088
Total, fair value 14,779 35,073
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses 0 0
Total unrealized gross losses 0 0
Less than 12 months, fair value 0 0
12 months or more, fair value 0 0
Total, fair value 0 0
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (694) (249)
12 months or more, unrealized gross losses (2,156) (2,373)
Total unrealized gross losses (2,850) (2,622)
Less than 12 months, fair value 87,482 116,173
12 months or more, fair value 196,342 235,846
Total, fair value 283,824 352,019
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (663) (165)
12 months or more, unrealized gross losses (49,002) (40,510)
Total unrealized gross losses (49,665) (40,675)
Less than 12 months, fair value 146,416 103,283
12 months or more, fair value 621,574 728,968
Total, fair value $ 767,990 $ 832,251