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Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Sep. 30, 2022
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost $ 5,873 $ 5,395
Gross Unrealized Gains 3 0
Gross Unrealized Losses (122) (134)
Fair Value 5,754 5,261
U.S. government-sponsored debt securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1,364 458
Gross Unrealized Gains 1 0
Gross Unrealized Losses (1) (1)
Fair Value 1,364 457
U.S. Treasury securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 4,509 4,937
Gross Unrealized Gains 2 0
Gross Unrealized Losses (121) (133)
Fair Value $ 4,390 $ 4,804