Equity and Noncontrolling Interests - Equity Attributable to Johnson Controls and Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Oct. 01, 2020	Oct. 01, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 17,656	$ 19,329	$ 17,447	$ 19,766
Beginning balance			1,086
Beginning balance			18,533
Net income (loss) attributable to Johnson Controls	343	213	794	372
Foreign currency translation adjustments	88	(471)	360	(220)
Foreign currency translation adjustments	63	(494)	371	(233)
Realized and unrealized gains (losses) on derivatives	8	(10)	9	(3)
Pension and postretirement plans	0	0	(1)	(1)
Other comprehensive income (loss)	71	(504)	379	(237)
Comprehensive income (loss) attributable to Johnson Controls	437	(268)	1,160	146
Comprehensive income attributable to noncontrolling interests	31	0	112	44
Total comprehensive income (loss)	468	(268)	1,272	190
Cash dividends - ordinary shares	(195)	(194)	(384)	(395)
Repurchases and retirements of ordinary shares	(315)	(816)	(661)	(1,467)
Change in noncontrolling interest share	(8)		(8)
Adoption of ASU	17,656	19,329	17,698	18,084
Adoption of ASU	1,059		1,086
Adoption of ASU	18,757		18,533
Other, including options exercised	123	33	148	39
Ending balance	17,698	18,084	17,698	18,084
Ending balance	1,059		1,059
Ending balance	18,757		18,757
Adoption of ASC 842 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				(5)
Adoption of ASU				(5)
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(4)
Adoption of ASU			(4)
Equity Attributable to Johnson Controls
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	17,656	19,329	17,447	19,766
Net income (loss) attributable to Johnson Controls	343	213	794	372
Foreign currency translation adjustments	88	(471)	360	(220)
Realized and unrealized gains (losses) on derivatives	6	(10)	7	(5)
Pension and postretirement plans			(1)	(1)
Other comprehensive income (loss)	94	(481)	366	(226)
Comprehensive income (loss) attributable to Johnson Controls	437	(268)	1,160	146
Cash dividends - ordinary shares	(195)	(194)	(384)	(395)
Dividends attributable to noncontrolling interests	0	0	0	0
Repurchases and retirements of ordinary shares	(315)	(816)	(661)	(1,467)
Change in noncontrolling interest share	(8)	0	(8)	0
Adoption of ASU	17,698	18,084	17,698	19,766
Other, including options exercised	123	33	148	39
Ending balance	17,698	18,084	17,698	18,084
Equity Attributable to Johnson Controls | Adoption of ASC 842 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU						$ (5)
Equity Attributable to Johnson Controls | Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU					$ (4)
Equity Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,167	1,102	1,086	1,063
Income attributable to noncontrolling interests	54	23	99	55
Foreign currency translation adjustments	(25)	(23)	11	(13)
Realized and unrealized gains on derivatives	2	0	2	2
Pension and postretirement plans			0	0
Other comprehensive income (loss)	(23)	(23)	13	(11)
Comprehensive income attributable to noncontrolling interests	31	0	112	44
Cash dividends - ordinary shares	0	0	0	0
Dividends attributable to noncontrolling interests	(133)	(98)	(133)	(103)
Repurchases and retirements of ordinary shares	0	0	0	0
Change in noncontrolling interest share	(6)	0	(6)	0
Adoption of ASU	1,059	1,004	1,086	1,063
Other, including options exercised	0	0	0	0
Ending balance	1,059	1,004	1,059	1,004
Equity Attributable to Noncontrolling Interest | Adoption of ASC 842 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		0
Adoption of ASU		0
Equity Attributable to Noncontrolling Interest | Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	0
Adoption of ASU	0
Total Equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	18,823	20,431	18,533	20,829
Net income	397	236	893	427
Foreign currency translation adjustments	63	(494)	371	(233)
Realized and unrealized gains (losses) on derivatives	8	(10)	9	(3)
Pension and postretirement plans			(1)	(1)
Other comprehensive income (loss)	71	(504)	379	(237)
Total comprehensive income (loss)	468	(268)	1,272	190
Cash dividends - ordinary shares	(195)	(194)	(384)	(395)
Dividends attributable to noncontrolling interests	(133)	(98)	(133)	(103)
Repurchases and retirements of ordinary shares	(315)	(816)	(661)	(1,467)
Change in noncontrolling interest share	(14)	0	(14)	0
Adoption of ASU	18,757	19,088	18,533	20,829
Other, including options exercised	123	33	148	39
Ending balance	$ 18,757	$ 19,088	$ 18,757	$ 19,088
Total Equity [Member] | Adoption of ASC 842 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU						$ (5)
Total Equity [Member] | Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU					$ (4)