Summary of Significant Accounting Policies Accounting Policies (Details) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) Segment $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Oct. 31, 2016 shares	Sep. 02, 2016 $ / shares
Financial Statement Details [Line Items]
Number Of Vies In Which Company Was Not Primary Beneficiary		3	2
Cash in escrow related to Adient debt				$ 2,000
Deferred tax asset from stock options exercised and restricted stock vestings	$ 179
Number of Countries in which Entity Operates		150
Maturity period to be considered cash equivalents		3 months
Restricted Cash and Cash Equivalents		$ 15	$ 31
Restricted Cash and Investments, Current		6	22
Restricted Cash and Cash Equivalents, Noncurrent		9	9
Costs and earnings in excess of billings related to contracts		1,054	908
Billing in excess of costs and earnings on uncompleted contracts		535	451
Liability for Asbestos and Environmental Claims, Gross		$ 550	573
Timing of the first deliverable to the last delivery		Approximately four to twelve months
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		$ 380	360	158
Foreign currency transaction gains (losses)		$ (5)	$ 94	(95)
Number of reportable segments | Segment		5
Proceeds from Issuance of Unsecured Debt				$ 2,000
Adient shares received per 10 shares of Johnson Controls shares | shares					1
Ordinary shares, par value | $ / shares		$ 0.01	$ 0.01			$ 1.00
Johnson Controls shares converted into one share of Adient | shares					10
Minimum
Financial Statement Details [Line Items]
Duration of extended warranty arrangements		1 year
Estimated useful life of dealer intangible assets		12 years
New Accounting Pronouncement, Effect of Change on Net Revenue, Percent		10.00%
Maximum
Financial Statement Details [Line Items]
Duration of extended warranty arrangements		5 years
Estimated useful life of dealer intangible assets		15 years
New Accounting Pronouncement, Effect of Change on Net Revenue, Percent		15.00%
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
Tyco merger [Member]
Financial Statement Details [Line Items]
Ordinary shares, par value | $ / shares						$ 1.00
ASU2016-16 [Member]
Financial Statement Details [Line Items]
New Accounting Pronouncement 2016-16, Effect of Change on Retained Earnings		$ 550
Accounting Standards Update 2014-09 [Member]
Financial Statement Details [Line Items]
New Accounting Pronouncement 2016-16, Effect of Change on Retained Earnings		$ 35