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Stockholders' Equity - Share Repurchases in the Open Market (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Class of Stock [Line Items]      
Total cost $ 8,114 $ 8,607 $ 7,192
Class A common stock      
Class of Stock [Line Items]      
Shares repurchased in the open market (in shares) 44 56 58
Average repurchase price per share (in dollars per share) $ 183.00 $ 154.01 $ 123.76
Total cost $ 8,114 $ 8,607 $ 7,192