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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

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