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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Sep. 30, 2022
--- | --- | ---
Amortized Cost, Available-for-sale securities |  |
Amortized Cost | $ 2,178,717 | $ 2,162,737
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains | 1,210 | 1,308
Gross unrealized loss | (120,090) | (113,008)
Fair Value, Available-for-sale securities |  |
Fair value | $ 2,059,837 | $ 2,051,037
Yield, Available-for-sale securities |  |
Yield | 3.99% | 3.36%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 453,443 | $ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 23 | 22
Held-to-maturity securities, gross unrealized losses | (51,411) | (56,461)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 402,055 | $ 406,860
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.88% | 2.88%
Investments | $ 2,632,160 | $ 2,626,036
Gross unrealized gains on investments | 1,233 | 1,330
Gross unrealized losses on investments | (171,501) | (169,469)
Total financial assets | $ 2,461,892 | $ 2,457,897
Yield on investments | 3.80% | 3.28%
U.S. government and agency securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 36,531 | $ 40,403
Amortized cost, over 10 years | 64,060 |
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 0
Gross unrealized gains, over 10 years | 223 |
Gross unrealized losses, 1 to 5 years | (925) | (1,049)
Gross unrealized losses, over 10 years | 0 |
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 35,606 | $ 39,354
Fair value, over 10 years | $ 64,283 |
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 4.33% | 3.03%
Yield, over 10 years | 3.90% |
Asset-backed securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 1 to 5 years | $ 21,033 | $ 22,527
Amortized cost, 5 to 10 years | 67,783 | 82,962
Amortized cost, over 10 years | 627,579 | 668,482
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 1 to 5 years | 0 | 0
Gross unrealized gains, 5 to 10 years | 0 | 4
Gross unrealized gains, over 10 years | 365 | 783
Gross unrealized losses, 1 to 5 years | (1,524) | (1,141)
Gross unrealized losses, 5 to 10 years | (1,025) | (547)
Gross unrealized losses, over 10 years | (17,639) | (6,069)
Fair Value, Available-for-sale securities |  |
Fair value, 1 to 5 years | 19,509 | 21,386
Fair value, 5 to 10 years | 66,758 | 82,419
Fair value, over 10 years | $ 610,305 | $ 663,196
Yield, Available-for-sale securities |  |
Yield, 1 to 5 years | 4.58% | 3.27%
Yield, 5 to 10 years | 5.01% | 3.53%
Yield, over 10 years | 5.12% | 3.86%
Corporate debt securities |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, within 1 year | $ 75,000 | $ 75,000
Amortized cost, 1 to 5 years | 151,531 | 151,411
Amortized cost, 5 to 10 years | 114,116 | 114,414
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, within 1 year | 29 | 4
Gross unrealized gains, 1 to 5 years | 232 | 0
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized losses, Within 1 year | (212) | (200)
Gross unrealized losses, 1 to 5 years | (3,435) | (2,748)
Gross unrealized losses, 5 to 10 years | (20,774) | (24,124)
Fair Value, Available-for-sale securities |  |
Fair value, within 1 year | 74,817 | 74,804
Fair value, 1 to 5 years | 148,328 | 148,663
Fair value, 5 to 10 years | $ 93,342 | $ 90,290
Yield, Available-for-sale securities |  |
Yield, within 1 year | 5.08% | 3.74%
Yield, 1 to 5 years | 4.49% | 3.59%
Yield, 5 to 10 years | 3.87% | 3.87%
Municipal bonds due |  |
Amortized Cost, Available-for-sale securities |  |
Amortized cost, 5 to 10 years | $ 5,743 | $ 5,751
Amortized cost, over 10 years | 29,861 | 29,871
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, 5 to 10 years | 0 | 0
Gross unrealized gains, over 10 years | 350 | 400
Gross unrealized losses, 5 to 10 years | (360) | (361)
Gross unrealized losses, over 10 years | (540) | (699)
Fair Value, Available-for-sale securities |  |
Fair value, 5 to 10 years | 5,383 | 5,390
Fair value, over 10 years | $ 29,671 | $ 29,572
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 | $ 985,480 | $ 971,916
Gross Unrealized Gains / Losses, Available-for-sale securities |  |
Gross unrealized gains, without single maturity date | 11 | 117
Gross unrealized losses, without single maturity date | (73,656) | (76,070)
Fair Value, Available-for-sale securities |  |
Fair value, without single maturity date | $ 911,835 | $ 895,963
Yield, Available-for-sale securities |  |
Yield, without single maturity date | 2.98% | 2.81%
Amortized Cost, Held-to-maturity securities |  |
Held-to-maturity securities | $ 453,443 | $ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities |  |
Held-to-maturity securities, gross unrealized gains | 23 | 22
Held-to-maturity securities, gross unrealized losses | (51,411) | (56,461)
Fair Value, Held-to-maturity securities |  |
Held-to-maturity securities, fair value | $ 402,055 | $ 406,860
Yield, Held-to-maturity securities |  |
Held-to-maturity securities, yield | 2.88% | 2.88%

X
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27198-111563 + Details
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