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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Mar. 31, 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 | 38 | 51
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (3,554) | $ (5,833)
12 months or more, unrealized gross losses | (1,369) | (3,781)
Total unrealized gross losses | (4,923) | (9,614)
Less than 12 months, fair value | 260,916 | 675,518
12 months or more, fair value | 157,632 | 285,161
Total, fair value | 418,548 | 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 | (3,554) | (5,885)
12 months or more, unrealized gross losses | (1,369) | (3,781)
Total unrealized gross losses | (4,923) | (9,666)
Less than 12 months, fair value | 260,916 | 682,371
12 months or more, fair value | 157,632 | 285,161
Total fair value | 418,548 | 967,532
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,514) | (74)
12 months or more, unrealized gross losses | 0 | (405)
Total unrealized gross losses | (1,514) | (479)
Less than 12 months, fair value | 125,191 | 45,875
12 months or more, fair value | 0 | 24,596
Total, fair value | 125,191 | 70,471
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (77) | (5,481)
12 months or more, unrealized gross losses | (1,058) | (3,027)
Total unrealized gross losses | (1,135) | (8,508)
Less than 12 months, fair value | 80,046 | 587,746
12 months or more, fair value | 125,681 | 204,369
Total, fair value | 205,727 | 792,115
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,963) | (278)
12 months or more, unrealized gross losses | (311) | (349)
Total unrealized gross losses | (2,274) | (627)
Less than 12 months, fair value | 55,679 | 41,897
12 months or more, fair value | 31,951 | 56,196
Total, fair value | $ 87,630 | $ 98,093

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