Acquisitions and Divestitures - Summary of Consideration Paid and Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition [Line Items]
Property, plant and equipment	$ 1	$ 2
Goodwill	5	24
Net assets acquired	$ 11	35
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period	9 years
Reacquired franchise rights
Business Acquisition [Line Items]
Intangible assets	$ 5	4
Customer relationships
Business Acquisition [Line Items]
Intangible assets	0	3
Trademarks and trade names
Business Acquisition [Line Items]
Intangible assets	0	1
Other
Business Acquisition [Line Items]
Intangible assets	$ 0	$ 1