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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Sep. 30, 2021
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 2,216,651 | $ 2,090,070
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 2,612 | 50,829
Gross unrealized loss | (68,531) | (2,640)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,150,732 | $ 2,138,259
Yield, Available-for-sale securities |  |
Yield | 2.49% | 1.69%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, amortized cost | $ 477,884 | $ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 115 | 13,522
Held-to-maturity securities, gross unrealized losses | (30,170) | 0
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 447,829 | $ 379,547
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.89% | 3.17%
Investments | $ 2,694,535 | $ 2,456,095
Gross unrealized gains on investments | 2,727 | 64,351
Gross unrealized losses on investments | (98,701) | (2,640)
Total financial assets | $ 2,598,561 | $ 2,517,806
Yield on investments | 2.63% | 1.89%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 46,584 | $ 47,339
Amortized cost, 5 to 10 years |  | 14,064
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 240
Gross unrealized gains, 5 to 10 years |  | 136
Gross unrealized losses, 1 to 5 years | (1,122) | 0
Gross unrealized losses, 5 to 10 years |  | 0
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | $ 45,462 | 47,579
Fair value, 5 to 10 years |  | $ 14,200
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 1.87% | 0.44%
Yield, 5 to 10 years |  | 2.05%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 23,396 | $ 19,730
Amortized cost, 5 to 10 years | 94,022 | 71,207
Amortized cost, over 10 years | 731,942 | 973,892
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 0
Gross unrealized gains, 5 to 10 years | 61 | 412
Gross unrealized gains, over 10 years | 780 | 14,069
Gross unrealized losses, 1 to 5 years | (1,066) | (434)
Gross unrealized losses, 5 to 10 years | (376) | (97)
Gross unrealized losses, over 10 years | (10,726) | (98)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 22,330 | 19,296
Fair value, 5 to 10 years | 93,707 | 71,522
Fair value, over 10 years | $ 721,996 | $ 987,863
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 2.00% | 0.52%
Yield, 5 to 10 years | 2.01% | 0.57%
Yield, over 10 years | 2.35% | 0.94%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 75,000 | $ 225,928
Amortized cost, 1 to 5 years | 151,290 | 86,802
Amortized cost, 5 to 10 years | 114,712 | 28,804
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 83 | 7,860
Gross unrealized gains, 1 to 5 years | 77 | 1,345
Gross unrealized gains, 5 to 10 years | 0 | 249
Gross unrealized losses, Within 1 year | 0 | 0
Gross unrealized losses, 1 to 5 years | (1,667) | 0
Gross unrealized losses, 5 to 10 years | (17,571) | 0
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 75,083 | 233,788
Fair value, 1 to 5 years | 149,700 | 88,147
Fair value, 5 to 10 years | $ 97,141 | $ 29,053
Yield, Available-for-sale securities |  |
Yield, within 1 year | 2.44% | 1.57%
Yield, 1 to 5 years | 2.68% | 4.64%
Yield, 5 to 10 years | 3.87% | 1.95%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 1,515 | $ 1,493
Amortized cost, 5 to 10 years | 5,758 | 5,781
Amortized cost, over 10 years | 29,880 | 29,909
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 0 | 17
Gross unrealized gains, 5 to 10 years | 0 | 294
Gross unrealized gains, over 10 years | 756 | 2,490
Gross unrealized losses, Within 1 year | 0 | 0
Gross unrealized losses, 5 to 10 years | (246) | 0
Gross unrealized losses, over 10 years | (432) | 0
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 1,515 | 1,510
Fair value, 5 to 10 years | 5,512 | 6,075
Fair value, over 10 years | $ 30,204 | $ 32,399
Yield, Available-for-sale securities |  |
Yield, within 1 year | 0.00% | 0.00%
Yield, 5 to 10 years | 0.16% | 0.22%
Yield, over 10 years | 5.85% | 5.85%
Agency pass-through certificates |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, without single maturity date | $ 942,552 | $ 585,121
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 855 | 23,717
Gross unrealized losses, without single maturity date | (35,325) | (2,011)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 908,082 | $ 606,827
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 2.60% | 2.60%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, amortized cost | $ 477,884 | $ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 115 | 13,522
Held-to-maturity securities, gross unrealized losses | (30,170) | 0
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 447,829 | $ 379,547
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.89% | 3.17%

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