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Fair Value Measurements and Investments - Contractual Maturity of Debt Securities (Detail) - USD ($)
$ in Millions
Sep. 30, 2019
Sep. 30, 2018
Fair Value Disclosures [Abstract]    
Long-term debt securities $ 2,157 $ 4,082
Fair Value    
Due within one year 4,110  
Due after 1 year through 5 years 2,157  
Fair Value $ 6,267 $ 7,516