Equity and Noncontrolling Interests Equity Attributable to Johnson Controls and Noncontrolling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019
Equity [Line Items]
Deferred tax asset from stock options exercised and restricted stock vestings	$ 179
Stock repurchase program, authorized amount					$ 8,500
Payments for Repurchase of Common Stock		$ 2,204	$ 5,983	$ 300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	20,447	19,766	21,164	20,447
Beginning balance		1,063
Beginning balance		20,829
Net income		631	5,674	2,162
Foreign currency translation adjustments		25	(342)	(483)
Realized and unrealized gains (losses) on derivatives		8	6	(29)
Realized and unrealized gains on marketable securities		0	0	4
Pension and postretirement plans		8	(6)	0
Other comprehensive income (loss)		41	(342)	(508)
Comprehensive income attributable to Johnson Controls		650	5,350	1,689
Comprehensive income attributable to noncontrolling interests		186	195	186
Total comprehensive income		836	5,545	1,875
Cash dividends - ordinary shares		(780)	(887)	(968)
Repurchases of ordinary shares			(5,983)	(300)
Other, including options exercised		(103)	(230)	(117)
Ending balance		17,447	19,766	$ 21,164
Ending balance		1,086	1,063
Ending balance		$ 18,533	$ 20,829
Cash dividends ordinary, per share		$ 1.04	$ 1.04	$ 1.04
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		$ (83)
Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	20,447	19,766	$ 21,164	$ 20,447
Net income		631	5,674	2,162
Foreign currency translation adjustments		7	(325)	(458)
Realized and unrealized gains (losses) on derivatives		4	7	(19)
Realized and unrealized gains on marketable securities				4
Pension and postretirement plans		8	(6)
Other comprehensive income (loss)		19	(324)	(473)
Comprehensive income attributable to Johnson Controls		650	5,350	1,689
Cash dividends - ordinary shares		(780)	(887)	(968)
Dividends attributable to noncontrolling interests		0	0	0
Repurchases of ordinary shares		(2,204)	(5,983)	(300)
Change in noncontrolling interest share				0
Noncontrolling Interest, Change in Redemption Value				0
Other, including options exercised		103	230	117
Ending balance		17,447	19,766	21,164
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(83)
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	920	1,063	1,294	920
Net income		164	213	186
Foreign currency translation adjustments		18	(17)	(22)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax, Noncontrolling Interest		4	(1)	(1)
Realized and unrealized gains on marketable securities				0
Pension and postretirement plans		0	0
Other comprehensive income (loss)		22	(18)	(23)
Comprehensive income attributable to noncontrolling interests		186	195	163
Cash dividends - ordinary shares		0	0	0
Dividends attributable to noncontrolling interests		(114)	(132)	(43)
Repurchases of ordinary shares		0	0	0
Change in noncontrolling interest share				23
Noncontrolling Interest, Change in Redemption Value				231
Other, including options exercised		0	1	0
Ending balance		1,086	1,063	1,294
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(49)
Stockholders' Equity, Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	21,367	20,829	22,458	21,367
Net Income		795	5,887	2,348
Foreign currency translation adjustments		25	(342)	(480)
Realized and unrealized gains (losses) on derivatives		8	6	(20)
Realized and unrealized gains on marketable securities				4
Pension and postretirement plans		8	(6)
Other comprehensive income (loss)		41	(342)	(496)
Total comprehensive income		836	5,545	1,852
Cash dividends - ordinary shares		(780)	(887)	(968)
Dividends attributable to noncontrolling interests		(114)	(132)	(43)
Repurchases of ordinary shares		(2,204)	(5,983)	(300)
Change in noncontrolling interest share				23
Noncontrolling Interest, Change in Redemption Value				231
Other, including options exercised		103	231	117
Ending balance		18,533	20,829	22,458
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(132)
Power Solutions
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Realized and unrealized gains (losses) on derivatives		0	4	0
Divestiture of Power Solutions			(483)
Power Solutions | Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Divestiture of Power Solutions			483
Power Solutions | Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Divestiture of Power Solutions			(295)
Power Solutions | Stockholders' Equity, Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Divestiture of Power Solutions			188
Adoption of ASC 606 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(45)
Ending balance				(45)
Adoption of ASC 606 | Parent | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(45)
Ending balance				(45)
Adoption of ASC 606 | Noncontrolling Interest [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			0
Ending balance				0
Adoption of ASC 606 | Stockholders' Equity, Total | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(45)
Ending balance				(45)
Adoption of ASC 842 | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(5)
Ending balance			(5)
Adoption of ASC 842 | Parent | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		(5)
Ending balance			(5)
Adoption of ASC 842 | Noncontrolling Interest [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		0
Ending balance			0
Adoption of ASC 842 | Stockholders' Equity, Total | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (5)
Ending balance			(5)
Accounting Standards Update 2016-09 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(179)			(179)
Accounting Standards Update 2016-09 [Member] | Parent | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	179			179
Accounting Standards Update 2016-09 [Member] | Noncontrolling Interest [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	0			0
Accounting Standards Update 2016-09 [Member] | Stockholders' Equity, Total | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 179			179
Accounting Standards Update 2016-16 Member [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(546)
Ending balance				(546)
Accounting Standards Update 2016-16 Member [Member] | Parent | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(546)
Ending balance				(546)
Accounting Standards Update 2016-16 Member [Member] | Noncontrolling Interest [Member] | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			0
Ending balance				0
Accounting Standards Update 2016-16 Member [Member] | Stockholders' Equity, Total | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ (546)
Ending balance				$ (546)