Basis of Presentation and Summary of Significant Accounting Policies - VIE (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Variable Interest Entity [Line Items]
Total assets	$ 308	$ 313
Total liabilities	257	252
Current assets
Variable Interest Entity [Line Items]
Total assets	266	270
Noncurrent assets
Variable Interest Entity [Line Items]
Total assets	42	43
Current liabilities
Variable Interest Entity [Line Items]
Total liabilities	$ 257	$ 252