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Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 547,008 | $ 533,762
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 16,203 | 22,797
Fair Value | 530,805 | 510,965
Money Market Funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 308,354 | 266,362
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 308,354 | 266,362
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 160,370 | 183,266
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 10,060 | 14,958
Fair Value | 150,310 | 168,308
U.S. Government Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 44,782 | 44,658
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 3,644 | 4,429
Fair Value | 41,138 | 40,229
Municipal Bond [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 7,513 | 17,759
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 417 | 1,634
Fair Value | 7,096 | 16,125
Supranational and Sovereign Debt [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 18,566 | 11,882
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 1,774 | 1,288
Fair Value | 16,792 | 10,594
Asset Backed Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 7,423 | 9,835
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 308 | 488
Fair Value | $ 7,115 | $ 9,347

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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://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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