Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Jul. 31, 2024
Financial Statement Details [Line Items]
Pooled subscriber assets and related deferred revenue, useful life	12 years
Non-pooled subscriber assets and related deferred revenue, useful life	15 years
Standard chargeback period from dealer for monitoring service cancellations	6 months
Non-standard chargeback period from dealer for monitoring service cancellations	12 months
Research and development expense	$ 273	$ 267	$ 251
Foreign currency transaction (losses) gains	$ (95)	(10)	$ 22
Maximum length, in years, of a product warranty for it to be recorded in other current liabilities	1 year
Minimum length, in years, of a product warranty for it to be recorded in other noncurrent liabilities	1 year
Accrual for best estimate of the probable loss on certain tax positions	$ 1,900
Accounts payable included in SCF programs	$ 835	$ 703
Supplier Finance Program, Obligation, Current, Statement of Financial Position [Extensible Enumeration]	Accounts payable	Accounts payable
Johnson Controls - Hitachi Air Conditioning Holding (UK) Ltd
Financial Statement Details [Line Items]
Joint venture ownership	60.00%			60.00%
Johnson Controls - Hitachi Air Conditioning Holding (UK) Ltd | Hitachi Global Life Solutions, Inc.
Financial Statement Details [Line Items]
Joint venture ownership	40.00%			40.00%
Restricted Stock
Financial Statement Details [Line Items]
Vesting period	3 years
Employee Stock Option
Financial Statement Details [Line Items]
Expiration period for stock options	10 years
Minimum
Financial Statement Details [Line Items]
Extended warranty arrangement duration	1 year
Deliverable period	4 months
Estimated useful life of dealer intangible assets	12 years
Negotiated commercial terms	90 days
Minimum | Performance Shares
Financial Statement Details [Line Items]
Vesting period	1 year
Minimum | Employee Stock Option
Financial Statement Details [Line Items]
Vesting period	2 years
Maximum
Financial Statement Details [Line Items]
Extended warranty arrangement duration	5 years
Deliverable period	24 months
Estimated useful life of dealer intangible assets	15 years
Negotiated commercial terms	120 days
Maximum | Performance Shares
Financial Statement Details [Line Items]
Vesting period	3 years
Maximum | Employee Stock Option
Financial Statement Details [Line Items]
Vesting period	3 years
Building And Improvements | Minimum
Financial Statement Details [Line Items]
Estimated useful lives	3 years
Building And Improvements | Maximum
Financial Statement Details [Line Items]
Estimated useful lives	40 years
Subscriber Systems | Maximum
Financial Statement Details [Line Items]
Estimated useful lives	15 years
Machinery and Equipment | Minimum
Financial Statement Details [Line Items]
Estimated useful lives	3 years
Machinery and Equipment | Maximum
Financial Statement Details [Line Items]
Estimated useful lives	15 years