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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Sep. 30, 2023
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized cost | $ 2,523,136 | $ 2,118,616
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 4,261 | 2,910
Gross unrealized loss | (89,283) | (126,429)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,438,114 | $ 1,995,097
Yield, Available-for-sale securities |  |
Yield | 4.76% | 4.64%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 457,882 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 23 | 0
Held-to-maturity securities, gross unrealized losses | (50,444) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 407,461 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.17% | 2.88%
Investments | $ 2,981,018 | $ 2,542,202
Gross unrealized gains on investments | 4,284 | 2,910
Gross unrealized losses on investments | (139,727) | (194,827)
Total financial assets | $ 2,845,575 | $ 2,350,285
Yield on investments | 4.47% | 4.35%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 8,692 | $ 3,501
Amortized cost, 1 to 5 years | 5,699 | 18,894
Amortized cost, 5 to 10 years | 162,027 | 87,922
Amortized cost, over 10 years | 135,961 | 106,340
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 5 | 0
Gross unrealized gains, 1 to 5 years | 4 | 0
Gross unrealized gains, 5 to 10 years | 524 | 177
Gross unrealized gains, over 10 years | 37 | 831
Gross unrealized losses, Within 1 year | (7) | (36)
Gross unrealized losses, 1 to 5 years | (264) | (563)
Gross unrealized losses, 5 to 10 years | (90) | 0
Gross unrealized losses, over 10 years | (389) | (13)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 8,690 | 3,465
Fair value, 1 to 5 years | 5,439 | 18,331
Fair value, 5 to 10 years | 162,461 | 88,099
Fair value, over 10 years | $ 135,609 | $ 107,158
Yield, Available-for-sale securities |  |
Yield, within 1 year | 5.83% | 6.06%
Yield, 1 to 5 years | 2.89% | 4.70%
Yield, 5 to 10 years | 6.01% | 5.76%
Yield, over 10 years | 6.32% | 5.84%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 13,249 | $ 18,579
Amortized cost, 5 to 10 years | 24,380 | 36,875
Amortized cost, over 10 years | 513,964 | 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 | 65 | 2
Gross unrealized gains, over 10 years | 1,251 | 578
Gross unrealized losses, 1 to 5 years | (397) | (715)
Gross unrealized losses, 5 to 10 years | 0 | (99)
Gross unrealized losses, over 10 years | (3,049) | (7,115)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 12,852 | 17,864
Fair value, 5 to 10 years | 24,445 | 36,778
Fair value, over 10 years | $ 512,166 | $ 533,374
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 6.08% | 6.06%
Yield, 5 to 10 years | 6.23% | 6.11%
Yield, over 10 years | 6.30% | 6.35%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 8,100 |
Amortized cost, 1 to 5 years | 144,034 | $ 151,893
Amortized cost, 5 to 10 years | 112,625 | 113,221
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 0 |
Gross unrealized gains, 1 to 5 years | 1,140 | 895
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized losses, Within 1 year | (1) |
Gross unrealized losses, 1 to 5 years | (845) | (1,787)
Gross unrealized losses, 5 to 10 years | (15,310) | (21,700)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 8,099 |
Fair value, 1 to 5 years | 144,329 | 151,001
Fair value, 5 to 10 years | $ 97,315 | $ 91,521
Yield, Available-for-sale securities |  |
Yield, within 1 year | 3.35% |
Yield, 1 to 5 years | 5.20% | 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 | $ 5,705 | $ 5,720
Amortized cost, over 10 years | 29,812 | 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 | 3 | 361
Gross unrealized losses, 5 to 10 years | (265) | (701)
Gross unrealized losses, over 10 years | (194) | (550)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 5,440 | 5,019
Fair value, over 10 years | $ 29,621 | $ 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 | $ 1,358,888 | $ 1,005,928
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 1,232 | 66
Gross unrealized losses, without single maturity date | (68,472) | (93,150)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 1,291,648 | $ 912,844
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 3.85% | 3.39%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 457,882 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 23 | 0
Held-to-maturity securities, gross unrealized losses | (50,444) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 407,461 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.17% | 2.88%

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