Fair Value Measurements (Carrying Amounts and Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions |
Mar. 31, 2017 |
Sep. 30, 2016 |
Mar. 31, 2016 |
Sep. 30, 2015 |
|---|---|---|---|---|
| Carrying Amounts and Fair Value of Financial Instruments | ||||
| Cash and cash equivalents | $ 281 | $ 340 | $ 300 | $ 252 |
| Long-term debt | (1,354) | (1,374) | ||
| Carrying Amount [Member] | ||||
| Carrying Amounts and Fair Value of Financial Instruments | ||||
| Cash and cash equivalents | 281 | 340 | ||
| Short-term Debt | (855) | (740) | ||
| Long-term debt | (1,339) | (1,339) | ||
| Fair Value [Member] | ||||
| Carrying Amounts and Fair Value of Financial Instruments | ||||
| Cash and cash equivalents | 281 | 340 | ||
| Short-term Debt | (855) | (740) | ||
| Long-term debt | $ (1,420) | $ (1,508) |