Summary of Significant Accounting Policies Accounting Policies (Details) $ in Millions	12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Financial Statement Details [Line Items]
Number of Countries in which Entity Operates	150
Restricted Cash and Investments, Current	$ 6	$ 9
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	$ 275	274	$ 319
Foreign currency transaction gains (losses)	$ 56	$ (32)	(10)
Minimum
Financial Statement Details [Line Items]
Estimated useful life of dealer intangible assets	12 years
Maximum
Financial Statement Details [Line Items]
Estimated useful life of dealer intangible assets	15 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
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
Power Solutions
Financial Statement Details [Line Items]
Disposal Group, Discontinued Operations, Consideration, net			$ 11,600	$ 11,600