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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Jun. 30, 2022 USD ($) security | Sep. 30, 2021 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 149 | 31
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (60,478) | $ (1,584)
12 months or more, unrealized gross losses | (8,053) | (1,056)
Total unrealized gross losses | (68,531) | (2,640)
Less than 12 months, fair value | 1,719,624 | 52,397
12 months or more, fair value | 135,304 | 102,324
Total, fair value | 1,854,928 | 154,721
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (30,170) | 0
Less than 12 months, fair value | 445,576 | 0
12 months or more, unrealized gross losses | 0 | 0
12 months or more, fair value | 0 | 0
Total unrealized gross losses | (30,170) | 0
Total, fair value | 445,576 | 0
Less than 12 months, unrealized gross losses | (90,648) | (1,584)
Less than 12 months, fair value | 2,165,200 | 52,397
12 months or more, unrealized gross losses | (8,053) | (1,056)
12 months or more, fair value | 135,304 | 102,324
Total unrealized gross losses | (98,701) | (2,640)
Total fair value | 2,300,504 | 154,721
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (19,238) |
12 months or more, unrealized gross losses | 0 |
Total unrealized gross losses | (19,238) |
Less than 12 months, fair value | 226,780 |
12 months or more, fair value | 0 |
Total, fair value | 226,780 |
Municipal bonds due |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (679) |
12 months or more, unrealized gross losses | 0 |
Total unrealized gross losses | (679) |
Less than 12 months, fair value | 14,960 |
12 months or more, fair value | 0 |
Total, fair value | 14,960 |
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,122) |
12 months or more, unrealized gross losses | 0 |
Total unrealized gross losses | (1,122) |
Less than 12 months, fair value | 45,462 |
12 months or more, fair value | 0 |
Total, fair value | 45,462 |
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (10,232) | (15)
12 months or more, unrealized gross losses | (1,935) | (614)
Total unrealized gross losses | (12,167) | (629)
Less than 12 months, fair value | 651,065 | 4,639
12 months or more, fair value | 77,426 | 70,058
Total, fair value | 728,491 | 74,697
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (29,207) | (1,569)
12 months or more, unrealized gross losses | (6,118) | (442)
Total unrealized gross losses | (35,325) | (2,011)
Less than 12 months, fair value | 781,357 | 47,758
12 months or more, fair value | 57,878 | 32,266
Total, fair value | $ 839,235 | $ 80,024

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