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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Sep. 30, 2023
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized cost | $ 2,101,314 | $ 2,118,616
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 2,075 | 2,910
Gross unrealized loss | (84,944) | (126,429)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,018,445 | $ 1,995,097
Yield, Available-for-sale securities |  |
Yield | 4.69% | 4.64%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 415,079 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 27 | 0
Held-to-maturity securities, gross unrealized losses | (44,164) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 370,942 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.88% | 2.88%
Investments | $ 2,516,393 | $ 2,542,202
Gross unrealized gains on investments | 2,102 | 2,910
Gross unrealized losses on investments | (129,108) | (194,827)
Total financial assets | $ 2,389,387 | $ 2,350,285
Yield on investments | 4.39% | 4.35%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 1,750 | $ 3,501
Amortized cost, 1 to 5 years | 16,747 | 18,894
Amortized cost, 5 to 10 years | 110,295 | 87,922
Amortized cost, over 10 years | 103,594 | 106,340
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 0 | 0
Gross unrealized gains, 1 to 5 years | 8 | 0
Gross unrealized gains, 5 to 10 years | 189 | 177
Gross unrealized gains, over 10 years | 267 | 831
Gross unrealized losses, Within 1 year | (5) | (36)
Gross unrealized losses, 1 to 5 years | (305) | (563)
Gross unrealized losses, 5 to 10 years | (173) | 0
Gross unrealized losses, over 10 years | (8) | (13)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 1,745 | 3,465
Fair value, 1 to 5 years | 16,450 | 18,331
Fair value, 5 to 10 years | 110,311 | 88,099
Fair value, over 10 years | $ 103,853 | $ 107,158
Yield, Available-for-sale securities |  |
Yield, within 1 year | 6.07% | 6.06%
Yield, 1 to 5 years | 4.54% | 4.70%
Yield, 5 to 10 years | 6.01% | 5.76%
Yield, over 10 years | 6.09% | 5.84%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 17,693 | $ 18,579
Amortized cost, 5 to 10 years | 31,469 | 36,875
Amortized cost, over 10 years | 519,103 | 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 | 14 | 2
Gross unrealized gains, over 10 years | 316 | 578
Gross unrealized losses, 1 to 5 years | (766) | (715)
Gross unrealized losses, 5 to 10 years | (205) | (99)
Gross unrealized losses, over 10 years | (6,320) | (7,115)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 16,927 | 17,864
Fair value, 5 to 10 years | 31,278 | 36,778
Fair value, over 10 years | $ 513,099 | $ 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 | 6.37% | 6.35%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 152,022 | $ 151,893
Amortized cost, 5 to 10 years | 112,923 | 113,221
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 479 | 895
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized losses, 1 to 5 years | (1,196) | (1,787)
Gross unrealized losses, 5 to 10 years | (15,415) | (21,700)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 151,305 | 151,001
Fair value, 5 to 10 years | $ 97,508 | $ 91,521
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 5.15% | 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 | $ 5,712 | $ 5,720
Amortized cost, over 10 years | 29,822 | 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 | 95 | 361
Gross unrealized losses, 5 to 10 years | (298) | (701)
Gross unrealized losses, over 10 years | (125) | (550)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 5,414 | 5,019
Fair value, over 10 years | $ 29,792 | $ 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 | $ 1,000,184 | $ 1,005,928
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 707 | 66
Gross unrealized losses, without single maturity date | (60,128) | (93,150)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 940,763 | $ 912,844
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 3.44% | 3.39%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 415,079 | $ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 27 | 0
Held-to-maturity securities, gross unrealized losses | (44,164) | (68,398)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 370,942 | $ 355,188
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.88% | 2.88%

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