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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Sep. 30, 2024
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized cost | $ 2,812,767 | $ 2,616,877
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 7,740 | 10,531
Gross unrealized loss | (76,776) | (54,699)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,743,731 | $ 2,572,709
Yield, Available-for-sale securities |  |
Yield | 4.67% | 4.87%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 537,348 | $ 436,972
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 13 | 913
Held-to-maturity securities, gross unrealized losses | (52,255) | (36,839)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 485,106 | $ 401,046
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.57% | 3.18%
Investments | $ 3,350,115 | $ 3,053,849
Gross unrealized gains on investments | 7,753 | 11,444
Gross unrealized losses on investments | (129,031) | (91,538)
Total financial assets | $ 3,228,837 | $ 2,973,755
Yield on investments | 4.41% | 4.63%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 2,184 | $ 4,360
Amortized cost, 1 to 5 years | 4,583 | 4,640
Amortized cost, 5 to 10 years | 195,082 | 166,070
Amortized cost, over 10 years | 90,460 | 137,799
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 1 | 4
Gross unrealized gains, 1 to 5 years | 1 | 2
Gross unrealized gains, 5 to 10 years | 1,327 | 1,230
Gross unrealized gains, over 10 years | 679 | 394
Gross unrealized losses, Within 1 year | (1) | 0
Gross unrealized losses, 1 to 5 years | (119) | (124)
Gross unrealized losses, 5 to 10 years | (4) | 0
Gross unrealized losses, over 10 years | 0 | (171)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 2,184 | 4,364
Fair value, 1 to 5 years | 4,465 | 4,518
Fair value, 5 to 10 years | 196,405 | 167,300
Fair value, over 10 years | $ 91,139 | $ 138,022
Yield, Available-for-sale securities |  |
Yield, within 1 year | 5.02% | 5.58%
Yield, 1 to 5 years | 2.69% | 2.82%
Yield, 5 to 10 years | 5.45% | 5.97%
Yield, over 10 years | 5.98% | 6.29%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 11,047 | $ 11,466
Amortized cost, 5 to 10 years | 8,026 | 9,631
Amortized cost, over 10 years | 528,444 | 520,756
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 0
Gross unrealized gains, 5 to 10 years | 13 | 0
Gross unrealized gains, over 10 years | 1,201 | 600
Gross unrealized losses, 1 to 5 years | (264) | (284)
Gross unrealized losses, 5 to 10 years | 0 | (3)
Gross unrealized losses, over 10 years | (1,685) | (2,041)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 10,783 | 11,182
Fair value, 5 to 10 years | 8,039 | 9,628
Fair value, over 10 years | $ 527,960 | $ 519,315
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 5.33% | 6.04%
Yield, 5 to 10 years | 5.48% | 6.20%
Yield, over 10 years | 5.60% | 6.15%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 45,011 | $ 45,024
Amortized cost, 1 to 5 years | 74,556 | 99,244
Amortized cost, 5 to 10 years | 161,731 | 112,029
Amortized cost, over 10 years |  | 50,000
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 0 | 0
Gross unrealized gains, 1 to 5 years | 507 | 977
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, over 10 years |  | 0
Gross unrealized losses, Within 1 year | (217) | (367)
Gross unrealized losses, 1 to 5 years | 0 | 0
Gross unrealized losses, 5 to 10 years | (13,714) | (10,625)
Gross unrealized losses, over 10 years |  | 0
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 44,794 | 44,657
Fair value, 1 to 5 years | 75,063 | 100,221
Fair value, 5 to 10 years | $ 148,017 | 101,404
Fair value, over 10 years |  | $ 50,000
Yield, Available-for-sale securities |  |
Yield, within 1 year | 4.61% | 4.61%
Yield, 1 to 5 years | 5.74% | 5.39%
Yield, 5 to 10 years | 4.57% | 3.87%
Yield, over 10 years |  | 6.85%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 5 to 10 years | $ 5,682 | $ 5,689
Amortized cost, over 10 years | 29,783 | 29,793
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, over 10 years | 17 | 0
Gross unrealized losses, 5 to 10 years | (375) | (243)
Gross unrealized losses, over 10 years | (314) | (166)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 5,307 | 5,446
Fair value, over 10 years | $ 29,486 | $ 29,627
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 | $ 1,656,178 | $ 1,420,376
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 3,994 | 7,324
Gross unrealized losses, without single maturity date | (60,083) | (40,675)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 1,600,089 | $ 1,387,025
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 4.15% | 4.09%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 537,348 | $ 436,972
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 13 | 913
Held-to-maturity securities, gross unrealized losses | (52,255) | (36,839)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 485,106 | $ 401,046
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.57% | 3.18%

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