Restructuring and Impairment Charges - Charges (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)
Restructuring and impairment charges
Restructuring charges		$ 28	$ 34	$ 41	$ 45
Number of reportable business segments | segment				4
Cost Smart Cost of Sales Program
Restructuring and impairment charges
Restructuring charges		14		$ 18
Cost Smart Cost of Sales Program | Accelerated depreciation
Restructuring and impairment charges
Restructuring charges			28		28
Cost Smart Cost of Sales Program | Employee-related severance costs
Restructuring and impairment charges
Restructuring charges			3		3
Cost Smart Cost of Sales Program, Production facility closure, Australia
Restructuring and impairment charges
Restructuring charges		6		6
Cost Smart Cost of Sales Program, Production facility closure, Australia | Forecast
Restructuring and impairment charges
Restructuring charges	$ 10
Cost Smart Cost of Sales Program, Production facility closure, Australia | Forecast | Minimum
Restructuring and impairment charges
Restructuring charges						$ 10
Cost Smart Cost of Sales Program, Production facility closure, Australia | Forecast | Maximum
Restructuring and impairment charges
Restructuring charges						$ 12
Cost Smart Cost of Sales Program, Production facility closure, Australia | Accelerated depreciation
Restructuring and impairment charges
Restructuring charges		2		2
Cost Smart Cost of Sales Program, Production facility closure, Australia | Employee-related severance costs
Restructuring and impairment charges
Restructuring charges		4		4
Cost Smart Cost of Sales Program, South America | Employee-related severance costs
Restructuring and impairment charges
Restructuring charges		4		4
Cost Smart Cost of Sales Program, North America
Restructuring and impairment charges
Restructuring charges		4		8
Cost Smart Cost of Sales Program, North America | Cessation of wet-milling
Restructuring and impairment charges
Restructuring charges				2
Expected additional costs		0		0
Cost Smart SG&A Program
Restructuring and impairment charges
Restructuring charges		14		23
Cost Smart SG&A Program | Employee-related severance costs
Restructuring and impairment charges
Restructuring charges		6		12
Cost Smart SG&A Program | Other restructuring costs
Restructuring and impairment charges
Restructuring charges		8		11
Cost Smart SG&A Program, Latin American Finance Transformation Initiative | Employee-related severance and other costs
Restructuring and impairment charges
Restructuring charges			$ 3		4
Cost Smart SG&A Program, Latin American Finance Transformation Initiative | Other restructuring costs
Restructuring and impairment charges
Restructuring charges		$ 1		$ 2
Cost Smart SG&A Program, North and South America | Employee-related severance costs
Restructuring and impairment charges
Restructuring charges					5
North America Finance Transformation Initiative | Other restructuring costs
Restructuring and impairment charges
Restructuring charges					4
Brazil leaf extraction Process restructuring | Other restructuring costs
Restructuring and impairment charges
Restructuring charges					$ 1