Acquisitions and Divestitures - Narrative (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($) business	Sep. 30, 2023 USD ($)	Jul. 31, 2025 USD ($)	Jul. 31, 2024
Business Acquisitions
Number of divestitures during period | business		3
Business divestitures, net of cash divested	$ 5	$ 345	$ 28
Net financing charges	0	(42)	0
Non-cash restructuring and impairment charges	427	411	$ 827
Total assets	$ 37,939	42,695
Johnson Controls - Hitachi Air Conditioning Holding (UK) Ltd
Business Acquisitions
Joint venture ownership	60.00%				60.00%
Johnson Controls - Hitachi Air Conditioning Holding (UK) Ltd | Hitachi Global Life Solutions, Inc.
Business Acquisitions
Joint venture ownership	40.00%				40.00%
Residential and Light Commercial HVAC Business | Discontinued Operations, Disposed of by Sale
Business Acquisitions
Disposal group, including discontinued operation, consideration				$ 5,600
Gain on sale of business, net of transaction and other costs, before tax	$ 2,700
Gain on sale of business, net of transaction and other costs, net of tax	$ 1,500
Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]	Income from discontinued operations, net of tax
Air Distribution Technologies (ADTi) | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Business Acquisitions
Disposal group, including discontinued operation, consideration		347
Business divestitures, net of cash divested		332
Net financing charges		42
Non-cash restructuring and impairment charges		$ 56