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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
security
Sep. 30, 2024
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 223 209
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (896) $ (429)
12 months or more, unrealized gross losses (75,879) (54,270)
Total unrealized gross losses (76,775) (54,699)
Less than 12 months, fair value 240,317 239,441
12 months or more, fair value 1,009,176 1,125,962
Total, fair value 1,249,493 1,365,403
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (805) 0
Less than 12 months, fair value 139,477 0
12 months or more, unrealized gross losses (51,451) (36,839)
12 months or more, fair value 324,133 348,573
Total unrealized gross losses (52,256) (36,839)
Total, fair value 463,610 348,573
Less than 12 months, unrealized gross losses (1,701) (429)
Less than 12 months, fair value 379,794 239,441
12 months or more, unrealized gross losses (127,330) (91,109)
12 months or more, fair value 1,333,309 1,474,535
Total unrealized gross losses (129,031) (91,538)
Total fair value 1,713,103 1,713,976
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 0
12 months or more, unrealized gross losses (13,931) (10,993)
Total unrealized gross losses (13,931) (10,993)
Less than 12 months, fair value 0 0
12 months or more, fair value 142,810 146,060
Total, fair value 142,810 146,060
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0 (15)
12 months or more, unrealized gross losses (689) (394)
Total unrealized gross losses (689) (409)
Less than 12 months, fair value 0 19,985
12 months or more, fair value 14,777 15,088
Total, fair value 14,777 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 (21) (249)
12 months or more, unrealized gross losses (2,052) (2,373)
Total unrealized gross losses (2,073) (2,622)
Less than 12 months, fair value 29,965 116,173
12 months or more, fair value 213,102 235,846
Total, fair value 243,067 352,019
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (875) (165)
12 months or more, unrealized gross losses (59,207) (40,510)
Total unrealized gross losses (60,082) (40,675)
Less than 12 months, fair value 210,352 103,283
12 months or more, fair value 638,487 728,968
Total, fair value $ 848,839 $ 832,251