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Fair Value Measurements and Investments - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 1,683	$ 2,405
Gross Unrealized Gains	14	16
Gross Unrealized Losses	0	0
Fair Value	1,697	2,421
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	179	304
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	180	305
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,504	2,101
Gross Unrealized Gains	13	15
Gross Unrealized Losses	0	0
Fair Value	$ 1,517	$ 2,116

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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).

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