Stockholders' Equity - Schedule of As-Converted Class B Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |
12 Months Ended | |
|---|---|---|
Sep. 30, 2019 |
Sep. 30, 2018 |
|
| Equity [Abstract] | ||
| Reduction in equivalent number of as-converted shares of class A common stock (in shares) | 2 | 5 |
| Effective price per share (in dollars per share) | $ 174.73 | $ 132.32 |
| Deposits under the U.S. retrospective responsibility plan | $ 300 | $ 600 |