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Fair Value Measurements and Investments - Schedule of Non-Marketable Equity Securities (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
Fair Value Disclosures [Abstract]        
Initial cost basis $ 858   $ 858  
Upward adjustments 355   355  
Downward adjustments (including impairment) (13)   (13)  
Carrying amount, end of period 1,200   1,200  
Upward adjustments 129 $ 0 143 $ 9
Downward adjustments $ 0 $ 0 $ (2) $ 0