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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Sep. 30, 2023
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized cost | $ 2,508,223 | $ 2,118,616
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 6,273 | 2,910
Gross unrealized loss | (85,727) | (126,429)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,428,769 | $ 1,995,097
Yield, Available-for-sale securities |  |
Yield | 4.81% | 4.64%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 447,638 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 170 | 0
Held-to-maturity securities, gross unrealized losses | (49,803) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 398,005 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.17% | 2.88%
Investments | $ 2,955,861 | $ 2,542,202
Gross unrealized gains on investments | 6,443 | 2,910
Gross unrealized losses on investments | (135,530) | (194,827)
Total financial assets | $ 2,826,774 | $ 2,350,285
Yield on investments | 4.51% | 4.35%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 6,530 | $ 3,501
Amortized cost, 1 to 5 years | 5,528 | 18,894
Amortized cost, 5 to 10 years | 151,376 | 87,922
Amortized cost, over 10 years | 128,092 | 106,340
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 3 | 0
Gross unrealized gains, 1 to 5 years | 3 | 0
Gross unrealized gains, 5 to 10 years | 1,205 | 177
Gross unrealized gains, over 10 years | 571 | 831
Gross unrealized losses, Within 1 year | (4) | (36)
Gross unrealized losses, 1 to 5 years | (233) | (563)
Gross unrealized losses, 5 to 10 years | 0 | 0
Gross unrealized losses, over 10 years | (296) | (13)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 6,529 | 3,465
Fair value, 1 to 5 years | 5,298 | 18,331
Fair value, 5 to 10 years | 152,581 | 88,099
Fair value, over 10 years | $ 128,367 | $ 107,158
Yield, Available-for-sale securities |  |
Yield, within 1 year | 5.84% | 6.06%
Yield, 1 to 5 years | 2.82% | 4.70%
Yield, 5 to 10 years | 6.01% | 5.76%
Yield, over 10 years | 6.33% | 5.84%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 12,369 | $ 18,579
Amortized cost, 5 to 10 years | 17,411 | 36,875
Amortized cost, over 10 years | 479,681 | 539,911
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 0
Gross unrealized gains, 5 to 10 years | 63 | 2
Gross unrealized gains, over 10 years | 672 | 578
Gross unrealized losses, 1 to 5 years | (334) | (715)
Gross unrealized losses, 5 to 10 years | 0 | (99)
Gross unrealized losses, over 10 years | (2,050) | (7,115)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 12,035 | 17,864
Fair value, 5 to 10 years | 17,474 | 36,778
Fair value, over 10 years | $ 478,303 | $ 533,374
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 6.10% | 6.06%
Yield, 5 to 10 years | 6.23% | 6.11%
Yield, over 10 years | 6.33% | 6.35%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 25,043 |
Amortized cost, 1 to 5 years | 119,108 | $ 151,893
Amortized cost, 5 to 10 years | 112,327 | 113,221
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 0 |
Gross unrealized gains, 1 to 5 years | 882 | 895
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized losses, Within 1 year | (436) |
Gross unrealized losses, 1 to 5 years | (163) | (1,787)
Gross unrealized losses, 5 to 10 years | (15,594) | (21,700)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 24,607 |
Fair value, 1 to 5 years | 119,827 | 151,001
Fair value, 5 to 10 years | $ 96,733 | $ 91,521
Yield, Available-for-sale securities |  |
Yield, within 1 year | 3.50% |
Yield, 1 to 5 years | 5.58% | 5.14%
Yield, 5 to 10 years | 3.87% | 3.87%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 5 to 10 years | $ 5,697 | $ 5,720
Amortized cost, over 10 years | 29,803 | 29,832
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, over 10 years | 274 | 361
Gross unrealized losses, 5 to 10 years | (514) | (701)
Gross unrealized losses, over 10 years | (243) | (550)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 5,183 | 5,019
Fair value, over 10 years | $ 29,834 | $ 29,643
Yield, Available-for-sale securities |  |
Yield, 5 to 10 years | 3.00% | 3.00%
Yield, over 10 years | 5.85% | 5.85%
Agency pass-through certificates |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, without single maturity date | $ 1,415,258 | $ 1,005,928
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 2,600 | 66
Gross unrealized losses, without single maturity date | (65,860) | (93,150)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 1,351,998 | $ 912,844
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 4.01% | 3.39%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 447,638 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 170 | 0
Held-to-maturity securities, gross unrealized losses | (49,803) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 398,005 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.17% | 2.88%

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