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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Dec. 31, 2023 USD ($) security | Sep. 30, 2023 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 220 | 231
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (253) | $ (4,185)
12 months or more, unrealized gross losses | (84,691) | (122,244)
Total unrealized gross losses | (84,944) | (126,429)
Less than 12 months, fair value | 152,930 | 243,952
12 months or more, fair value | 1,481,661 | 1,393,013
Total, fair value | 1,634,591 | 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 | (44,164) | (68,383)
12 months or more, fair value | 369,265 | 353,764
Total unrealized gross losses | (44,164) | (68,398)
Total, fair value | 369,265 | 355,188
Less than 12 months, unrealized gross losses | (253) | (4,200)
Less than 12 months, fair value | 152,930 | 245,376
12 months or more, unrealized gross losses | (128,855) | (190,627)
12 months or more, fair value | 1,850,926 | 1,746,777
Total unrealized gross losses | (129,108) | (194,827)
Total fair value | 2,003,856 | 1,992,153
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | 0 | 0
12 months or more, unrealized gross losses | (16,611) | (23,487)
Total unrealized gross losses | (16,611) | (23,487)
Less than 12 months, fair value | 0 | 0
12 months or more, fair value | 174,074 | 167,452
Total, fair value | 174,074 | 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 | (423) | (1,250)
Total unrealized gross losses | (423) | (1,250)
Less than 12 months, fair value | 0 | 0
12 months or more, fair value | 15,111 | 14,302
Total, fair value | 15,111 | 14,302
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (181) | (13)
12 months or more, unrealized gross losses | (310) | (599)
Total unrealized gross losses | (491) | (612)
Less than 12 months, fair value | 134,687 | 14,917
12 months or more, fair value | 13,720 | 21,795
Total, fair value | 148,407 | 36,712
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | 0 | (2,142)
12 months or more, unrealized gross losses | (7,291) | (5,788)
Total unrealized gross losses | (7,291) | (7,930)
Less than 12 months, fair value | 0 | 86,800
12 months or more, fair value | 522,583 | 445,454
Total, fair value | 522,583 | 532,254
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (72) | (2,030)
12 months or more, unrealized gross losses | (60,056) | (91,120)
Total unrealized gross losses | (60,128) | (93,150)
Less than 12 months, fair value | 18,243 | 142,235
12 months or more, fair value | 756,173 | 744,010
Total, fair value | $ 774,416 | $ 886,245

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