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Fair Value Measurements - Summary of Investment Portfolio (Parenthetical) (Details) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Mar. 31, 2022	Sep. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds restricted for specific transactions	[1]	$ 44	$ 91
Environmental Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds restricted for specific transactions		$ 90	$ 90
[1] The March 31, 2022 period included additional contributions to the Environmental trust from proceeds associated with the Performance Adhesives sale and excess land sales. The September 30, 2021 period included $90 million to establish the Environmental trust.

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