Recent Accounting Pronouncements - Narrative (Details) - USD ($) $ in Millions |
2 Months Ended | 10 Months Ended | 12 Months Ended | |
|---|---|---|---|---|
Dec. 15, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
Sep. 30, 2016 |
|
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Other pension and post-retirement benefit credits (costs), net | $ 13 | |||
| Restricted cash | 4 | |||
| Accounting Standards Update 2016-18 | ||||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Restricted cash | $ 4 | |||
| Predecessor | ||||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Other pension and post-retirement benefit credits (costs), net | $ (8) | $ (34) | $ (27) | |
| Predecessor | Accounting Standards Update 2016-18 | ||||
| New Accounting Pronouncements or Change in Accounting Principle [Line Items] | ||||
| Restricted cash | $ 91 | |||