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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Mar. 31, 2022 USD ($) security | Sep. 30, 2021 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 131 | 31
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (7,667) | $ (1,584)
12 months or more, unrealized gross losses | (15,704) | (1,056)
Total unrealized gross losses | (23,371) | (2,640)
Less than 12 months, fair value | 551,598 | 52,397
12 months or more, fair value | 236,765 | 102,324
Total, fair value | 788,363 | 154,721
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (7,037) | 0
Less than 12 months, fair value | 291,911 | 0
12 months or more, unrealized gross losses | 0 | 0
12 months or more, fair value | 0 | 0
Total unrealized gross losses | (7,037) | 0
Total, fair value | 291,911 | 0
Less than 12 months, unrealized gross losses | (14,704) | (1,584)
Less than 12 months, fair value | 843,509 | 52,397
12 months or more, unrealized gross losses | (15,704) | (1,056)
12 months or more, fair value | 236,765 | 102,324
Total unrealized gross losses | (30,408) | (2,640)
Total fair value | 1,080,274 | 154,721
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | 0 |
12 months or more, unrealized gross losses | (9,482) |
Total unrealized gross losses | (9,482) |
Less than 12 months, fair value | 0 |
12 months or more, fair value | 105,528 |
Total, fair value | 105,528 |
Municipal bonds due |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (156) |
12 months or more, unrealized gross losses | 0 |
Total unrealized gross losses | (156) |
Less than 12 months, fair value | 5,610 |
12 months or more, fair value | 0 |
Total, fair value | 5,610 |
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (659) |
12 months or more, unrealized gross losses | 0 |
Total unrealized gross losses | (659) |
Less than 12 months, fair value | 34,697 |
12 months or more, fair value | 0 |
Total, fair value | 34,697 |
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,269) | (15)
12 months or more, unrealized gross losses | (972) | (614)
Total unrealized gross losses | (2,241) | (629)
Less than 12 months, fair value | 277,125 | 4,639
12 months or more, fair value | 67,792 | 70,058
Total, fair value | 344,917 | 74,697
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (5,583) | (1,569)
12 months or more, unrealized gross losses | (5,250) | (442)
Total unrealized gross losses | (10,833) | (2,011)
Less than 12 months, fair value | 234,166 | 47,758
12 months or more, fair value | 63,445 | 32,266
Total, fair value | $ 297,611 | $ 80,024

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