Equity and Noncontrolling Interests Equity Attributable to Johnson Controls and Noncontrolling Interests (Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Increase in Authorized Amount	$ 4,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 5,100			$ 2,000
Repurchases and retirements of ordinary shares		1,307	$ 2,204	$ 5,983
Stock Repurchased and Retired During Period Value Tender Offer				4,035
Stock Repurchased and Retired During Period Value Open Market				1,948
Parent
Equity, Class of Treasury Stock [Line Items]
Repurchases and retirements of ordinary shares		1,307	2,204	5,983
Stockholders' Equity, Total
Equity, Class of Treasury Stock [Line Items]
Repurchases and retirements of ordinary shares		$ 1,307	$ 2,204	$ 5,983