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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Jun. 30, 2024 USD ($) security | Sep. 30, 2023 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 227 | 231
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (1,288) | $ (4,185)
12 months or more, unrealized gross losses | (84,440) | (122,244)
Total unrealized gross losses | (85,728) | (126,429)
Less than 12 months, fair value | 311,190 | 243,952
12 months or more, fair value | 1,132,369 | 1,393,013
Total, fair value | 1,443,559 | 1,636,965
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | 0 | (15)
Less than 12 months, fair value | 0 | 1,424
12 months or more, unrealized gross losses | (49,802) | (68,383)
12 months or more, fair value | 345,952 | 353,764
Total unrealized gross losses | (49,802) | (68,398)
Total, fair value | 345,952 | 355,188
Less than 12 months, unrealized gross losses | (1,288) | (4,200)
Less than 12 months, fair value | 311,190 | 245,376
12 months or more, unrealized gross losses | (134,242) | (190,627)
12 months or more, fair value | 1,478,321 | 1,746,777
Total unrealized gross losses | (135,530) | (194,827)
Total fair value | 1,789,511 | 1,992,153
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (163) | 0
12 months or more, unrealized gross losses | (16,030) | (23,487)
Total unrealized gross losses | (16,193) | (23,487)
Less than 12 months, fair value | 19,831 | 0
12 months or more, fair value | 121,341 | 167,452
Total, fair value | 141,172 | 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 | (757) | (1,250)
Total unrealized gross losses | (757) | (1,250)
Less than 12 months, fair value | 0 | 0
12 months or more, fair value | 14,743 | 14,302
Total, fair value | 14,743 | 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 | (336) | (2,142)
12 months or more, unrealized gross losses | (2,583) | (5,788)
Total unrealized gross losses | (2,919) | (7,930)
Less than 12 months, fair value | 59,540 | 86,800
12 months or more, fair value | 299,752 | 445,454
Total, fair value | 359,292 | 532,254
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (789) | (2,030)
12 months or more, unrealized gross losses | (65,070) | (91,120)
Total unrealized gross losses | (65,859) | (93,150)
Less than 12 months, fair value | 231,819 | 142,235
12 months or more, fair value | 696,533 | 744,010
Total, fair value | $ 928,352 | $ 886,245

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