Stockholders' Equity - Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Sep. 30, 2019 |
Sep. 30, 2018 |
Sep. 30, 2017 |
|
| Conversion of Stock [Line Items] | |||
| Recovery through conversion rate adjustment | $ 8 | ||
| Series B Preferred Stock | |||
| Conversion of Stock [Line Items] | |||
| Reduction in equivalent number of as-converted shares of class A common stock | 0 | 0 | 2 |
| Effective price per share (in dollars per share) | $ 141.32 | $ 113.05 | $ 88.70 |
| Recovery through conversion rate adjustment | $ 6 | $ 35 | $ 190 |
| Series C Preferred Stock | |||
| Conversion of Stock [Line Items] | |||
| Reduction in equivalent number of as-converted shares of class A common stock | 0 | 0 | 0 |
| Effective price per share (in dollars per share) | $ 150.26 | $ 112.92 | $ 85.01 |
| Recovery through conversion rate adjustment | $ 2 | $ 21 | $ 1 |