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Investment Securities - (Unrealized Losses and Fair Value of Securities) (Details) $ in Thousands | Sep. 30, 2022 USD ($) security | Sep. 30, 2021 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 223 | 31
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (99,294) | $ (1,584)
12 months or more, unrealized gross losses | (13,714) | (1,056)
Total, unrealized gross losses | (113,008) | (2,640)
Less than 12 months, fair value | 1,777,599 | 52,397
12 months or more, fair value | 103,603 | 102,324
Total, fair value | 1,881,202 | 154,721
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (56,461) | 0
12 months or more, unrealized gross losses | 0 | 0
Total unrealized gross losses | (56,461) | 0
Less than 12 months, fair value | 405,166 | 0
12 months or more, fair value | 0 | 0
Total, fair value | 405,166 | 0
Less than 12 months, unrealized gross losses | (155,755) | (1,584)
12 months or more, unrealized gross losses | (13,714) | (1,056)
Total unrealized gross losses | (169,469) | (2,640)
Less than 12 months, fair value | 2,182,765 | 52,397
12 months or more, fair value | 103,603 | 102,324
Total fair value | 2,286,368 | 154,721
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (27,072) |
12 months or more, unrealized gross losses | 0 |
Total, unrealized gross losses | (27,072) |
Less than 12 months, fair value | 288,753 |
12 months or more, fair value | 0 |
Total, fair value | 288,753 |
Municipal bonds |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,061) |
12 months or more, unrealized gross losses | 0 |
Total, unrealized gross losses | (1,061) |
Less than 12 months, fair value | 14,561 |
12 months or more, fair value | 0 |
Total, fair value | 14,561 |
U.S. government and agency securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,049) |
12 months or more, unrealized gross losses | 0 |
Total, unrealized gross losses | (1,049) |
Less than 12 months, fair value | 39,354 |
12 months or more, fair value | 0 |
Total, fair value | 39,354 |
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (6,374) | (15)
12 months or more, unrealized gross losses | (1,383) | (614)
Total, unrealized gross losses | (7,757) | (629)
Less than 12 months, fair value | 601,248 | 4,639
12 months or more, fair value | 50,070 | 70,058
Total, fair value | 651,318 | 74,697
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (63,738) | (1,569)
12 months or more, unrealized gross losses | (12,331) | (442)
Total, unrealized gross losses | (76,069) | (2,011)
Less than 12 months, fair value | 833,683 | 47,758
12 months or more, fair value | 53,533 | 32,266
Total, fair value | $ 887,216 | $ 80,024

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