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Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 2022 | Sep. 30, 2021
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 533,762 | $ 466,827
Gross Unrealized Gains | 0 | 80
Gross Unrealized Losses | 22,797 | 1,354
Fair Value | 510,965 | 465,553
Money Market Funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 266,362 | 209,026
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 266,362 | 209,026
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 183,266 | 191,445
Gross Unrealized Gains | 0 | 76
Gross Unrealized Losses | 14,958 | 1,084
Fair Value | 168,308 | 190,437
U.S. Government Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 44,658 | 54,987
Gross Unrealized Gains | 0 | 4
Gross Unrealized Losses | 4,429 | 239
Fair Value | 40,229 | 54,752
Municipal Bond [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 17,759 |
Gross Unrealized Losses | 1,634 |
Fair Value | 16,125 |
Supranational and Sovereign Debt [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 11,882 | 7,479
Gross Unrealized Losses | 1,288 | 26
Fair Value | 10,594 | 7,453
Asset Backed Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,835 | 3,890
Gross Unrealized Losses | 488 | 5
Fair Value | $ 9,347 | $ 3,885

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563 + Details
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