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Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 5,806 | $ 5,395
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (82) | (134)
Fair Value | 5,725 | 5,261
U.S. government-sponsored debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,109 | 458
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (2) | (1)
Fair Value | 1,108 | 457
U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,697 | 4,937
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (80) | (133)
Fair Value | $ 4,617 | $ 4,804

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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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