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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Mar. 31, 2024 USD ($) security | Sep. 30, 2023 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 253 | 231
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (1,433) | $ (4,185)
12 months or more, unrealized gross losses | (87,850) | (122,244)
Total unrealized gross losses | (89,283) | (126,429)
Less than 12 months, fair value | 338,990 | 243,952
12 months or more, fair value | 1,137,460 | 1,393,013
Total, fair value | 1,476,450 | 1,636,965
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (2) | (15)
Less than 12 months, fair value | 263 | 1,424
12 months or more, unrealized gross losses | (50,442) | (68,383)
12 months or more, fair value | 354,819 | 353,764
Total unrealized gross losses | (50,444) | (68,398)
Total, fair value | 355,082 | 355,188
Less than 12 months, unrealized gross losses | (1,435) | (4,200)
Less than 12 months, fair value | 339,253 | 245,376
12 months or more, unrealized gross losses | (138,292) | (190,627)
12 months or more, fair value | 1,492,279 | 1,746,777
Total unrealized gross losses | (139,727) | (194,827)
Total fair value | 1,831,532 | 1,992,153
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (214) | 0
12 months or more, unrealized gross losses | (15,942) | (23,487)
Total unrealized gross losses | (16,156) | (23,487)
Less than 12 months, fair value | 19,779 | 0
12 months or more, fair value | 154,496 | 167,452
Total, fair value | 174,275 | 167,452
Municipal bonds due |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | 0 | 0
12 months or more, unrealized gross losses | (458) | (1,250)
Total unrealized gross losses | (458) | (1,250)
Less than 12 months, fair value | 0 | 0
12 months or more, fair value | 15,059 | 14,302
Total, fair value | 15,059 | 14,302
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses |  | (13)
12 months or more, unrealized gross losses |  | (599)
Total unrealized gross losses |  | (612)
Less than 12 months, fair value |  | 14,917
12 months or more, fair value |  | 21,795
Total, fair value |  | 36,712
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (485) | (2,142)
12 months or more, unrealized gross losses | (3,712) | (5,788)
Total unrealized gross losses | (4,197) | (7,930)
Less than 12 months, fair value | 110,515 | 86,800
12 months or more, fair value | 247,943 | 445,454
Total, fair value | 358,458 | 532,254
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (734) | (2,030)
12 months or more, unrealized gross losses | (67,738) | (91,120)
Total unrealized gross losses | (68,472) | (93,150)
Less than 12 months, fair value | 208,696 | 142,235
12 months or more, fair value | 719,962 | 744,010
Total, fair value | $ 928,658 | $ 886,245

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