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Investment Securities - (Unrealized Losses and Fair Value of Securities) (Details) $ in Thousands | Sep. 30, 2021 USD ($) security | Sep. 30, 2020 USD ($) security
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Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 31 | 51
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (1,584) | $ (5,833)
12 months or more, unrealized gross losses | (1,056) | (3,781)
Total, unrealized gross losses | (2,640) | (9,614)
Less than 12 months, fair value | 52,397 | 675,518
12 months or more, fair value | 102,324 | 285,161
Total, fair value | 154,721 | 960,679
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | 0 | (52)
12 months or more, unrealized gross losses | 0 | 0
Total unrealized gross losses | 0 | (52)
Less than 12 months, fair value | 0 | 6,853
12 months or more, fair value | 0 | 0
Total, fair value | 0 | 6,853
Less than 12 months, unrealized gross losses | (1,584) | (5,885)
12 months or more, unrealized gross losses | (1,056) | (3,781)
Total unrealized gross losses | (2,640) | (9,666)
Less than 12 months, fair value | 52,397 | 682,371
12 months or more, fair value | 102,324 | 285,161
Total fair value | 154,721 | 967,532
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses |  | (74)
12 months or more, unrealized gross losses |  | (405)
Total, unrealized gross losses |  | (479)
Less than 12 months, fair value |  | 45,875
12 months or more, fair value |  | 24,596
Total, fair value |  | 70,471
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (15) | (5,481)
12 months or more, unrealized gross losses | (614) | (3,027)
Total, unrealized gross losses | (629) | (8,508)
Less than 12 months, fair value | 4,639 | 587,746
12 months or more, fair value | 70,058 | 204,369
Total, fair value | 74,697 | 792,115
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,569) | (278)
12 months or more, unrealized gross losses | (442) | (349)
Total, unrealized gross losses | (2,011) | (627)
Less than 12 months, fair value | 47,758 | 41,897
12 months or more, fair value | 32,266 | 56,196
Total, fair value | $ 80,024 | $ 98,093

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