Basis of Presentation and Summary of Significant Accounting Policies - VIE (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Variable Interest Entity [Line Items]
Total assets	$ 263	$ 313
Total liabilities	197	252
Current assets
Variable Interest Entity [Line Items]
Total assets	221	270
Noncurrent assets
Variable Interest Entity [Line Items]
Total assets	42	43
Current liabilities
Variable Interest Entity [Line Items]
Total liabilities	$ 197	$ 252