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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Sep. 30, 2020
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 2,392,676 | $ 2,210,099
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 51,149 | 49,007
Gross unrealized loss | (4,923) | (9,614)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,438,902 | $ 2,249,492
Yield, Available-for-sale securities |  |
Yield | 1.80% | 1.97%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, amortized cost | $ 494,089 | $ 705,838
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 13,808 | 21,582
Held-to-maturity securities, gross unrealized losses | 0 | (52)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 507,897 | $ 727,368
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.14% | 3.14%
Investments | $ 2,886,765 | $ 2,915,937
Gross unrealized gains on investments | 64,957 | 70,589
Gross unrealized losses on investments | (4,923) | (9,666)
Total financial assets | $ 2,946,799 | $ 2,976,860
Yield on investments | 2.02% | 2.25%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 54,992 |
Amortized cost, 5 to 10 years | 15,897 | $ 18,448
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 516 |
Gross unrealized gains, 5 to 10 years | 107 | 376
Gross unrealized losses, 1 to 5 years | 0 |
Gross unrealized losses, 5 to 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 55,508 |
Fair value, 5 to 10 years | $ 16,004 | $ 18,824
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 0.50% |
Yield, 5 to 10 years | 2.05% | 2.05%
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 5 to 10 years | $ 102,141 | $ 38,289
Amortized cost, over 10 years | 1,071,530 | 906,489
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 5 to 10 years | 447 | 0
Gross unrealized gains, over 10 years | 10,994 | 647
Gross unrealized losses, 5 to 10 years | (718) | (1,600)
Gross unrealized losses, over 10 years | (417) | (6,908)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 101,870 | 36,689
Fair value, over 10 years | $ 1,082,107 | $ 900,228
Yield, Available-for-sale securities |  |
Yield, 5 to 10 years | 0.68% | 0.83%
Yield, over 10 years | 1.07% | 1.14%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 31,742 | $ 54,209
Amortized cost, 1 to 5 years | 195,442 | 128,289
Amortized cost, 5 to 10 years | 65,888 | 97,157
Amortized cost, over 10 years | 80,560 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 126 | 337
Gross unrealized gains, 1 to 5 years | 7,120 | 3,366
Gross unrealized gains, 5 to 10 years | 751 | 4,305
Gross unrealized gains, over 10 years | 0 |
Gross unrealized losses, Within 1 year | (28) | (51)
Gross unrealized losses, 1 to 5 years | 0 | (428)
Gross unrealized losses, 5 to 10 years | (710) | 0
Gross unrealized losses, over 10 years | (776) |
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 31,840 | 54,495
Fair value, 1 to 5 years | 202,562 | 131,227
Fair value, 5 to 10 years | 65,929 | $ 101,462
Fair value, over 10 years | $ 79,784 |
Yield, Available-for-sale securities |  |
Yield, within 1 year | 1.98% | 1.22%
Yield, 1 to 5 years | 1.67% | 1.78%
Yield, 5 to 10 years | 3.14% | 1.50%
Yield, over 10 years | 3.44% |
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 1,477 | $ 1,461
Amortized cost, 5 to 10 years | 5,797 |
Amortized cost, over 10 years | 29,929 | 36,044
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 28 | 36
Gross unrealized gains, 5 to 10 years | 275 |
Gross unrealized gains, over 10 years | 1,163 | 774
Gross unrealized losses, 1 to 5 years | 0 | 0
Gross unrealized losses, 5 to 10 years | 0 |
Gross unrealized losses, over 10 years | 0 | 0
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 1,505 | 1,497
Fair value, 5 to 10 years | 6,072 |
Fair value, over 10 years | $ 31,092 | $ 36,818
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 0.00% | 0.00%
Yield, 5 to 10 years | 0.27% |
Yield, over 10 years | 5.85% | 5.40%
Agency pass-through certificates |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, without single maturity date | $ 737,281 | $ 929,713
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 29,622 | 39,166
Gross unrealized losses, without single maturity date | (2,274) | (627)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 764,629 | $ 968,252
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 2.67% | 2.82%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, amortized cost | $ 489,915 | $ 698,934
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 13,808 | 21,582
Held-to-maturity securities, gross unrealized losses | 0 | 0
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 503,723 | $ 720,516
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 3.16% | 3.16%
Commercial MBS |  |
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities, amortized cost | $ 4,174 | $ 6,904
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 0 | 0
Held-to-maturity securities, gross unrealized losses | 0 | (52)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 4,174 | $ 6,852
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 0.98% | 1.02%

X
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