Equity and Noncontrolling Interests - Equity Attributable to Johnson Controls and Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 19,329	$ 20,102	$ 21,164	$ 19,766	$ 21,164
Beginning balance				1,063
Beginning balance				20,829
Net income (loss) attributable to Johnson Controls	213	515		372	870
Foreign currency translation adjustments	(471)	114		(220)	(13)
Foreign currency translation adjustments	(494)	117		(233)	(1)
Realized and unrealized gains (losses) on derivatives	(10)	17		(3)	19
Pension and postretirement plans	0	0		(1)	0
Other comprehensive income (loss)	(504)	134		(237)	18
Comprehensive income (loss) attributable to Johnson Controls	(268)	643		146	872
Comprehensive income attributable to noncontrolling interests	0	49		44	103
Total comprehensive income (loss)	(268)	692		190	975
Cash dividends - ordinary shares	(194)	(235)		(395)	(475)
Stock Repurchased and Retired During Period, Value	(816)			(1,467)
Repurchases of ordinary shares		(533)			(1,000)
Other, including options exercised	33	59		39	66
Ending balance	18,084	20,036	20,102	18,084	20,036
Ending balance	1,004			1,004
Ending balance	19,088			19,088
Equity Attributable to Johnson Controls
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	19,329	20,102	21,164	19,766	21,164
Net income (loss) attributable to Johnson Controls	213	515		372	870
Foreign currency translation adjustments	(471)	114		(220)	(13)
Realized and unrealized gains (losses) on derivatives	(10)	14		(5)	15
Pension and postretirement plans				(1)	0
Other comprehensive income (loss)	(481)	128		(226)	2
Comprehensive income (loss) attributable to Johnson Controls	(268)	643		146	872
Cash dividends - ordinary shares	(194)	(235)		(395)	(475)
Dividends attributable to noncontrolling interests	0	0		0	0
Stock Repurchased and Retired During Period, Value	(816)			(1,467)
Repurchases of ordinary shares		(533)			(1,000)
Other, including options exercised	33	59		39	66
Ending balance	18,084	20,036	20,102	18,084	20,036
Equity Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,102	1,305	1,294	1,063	1,294
Income attributable to noncontrolling interests	23	43		55	87
Foreign currency translation adjustments	(23)	3		(13)	12
Realized and unrealized gains on derivatives	0	3		2	4
Pension and postretirement plans				0	0
Other comprehensive income (loss)	(23)	6		(11)	16
Comprehensive income attributable to noncontrolling interests	0	49		44	103
Cash dividends - ordinary shares	0	0		0	0
Dividends attributable to noncontrolling interests	(98)	(89)		(103)	(132)
Stock Repurchased and Retired During Period, Value	0			0
Repurchases of ordinary shares		0			0
Other, including options exercised	0	0		0	0
Ending balance	1,004	1,265	1,305	1,004	1,265
Total Equity Excluding Redeemable Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	20,431	21,407	22,458	20,829	22,458
Net income	236	558		427	957
Foreign currency translation adjustments	(494)	117		(233)	(1)
Realized and unrealized gains (losses) on derivatives	(10)	17		(3)	19
Pension and postretirement plans				(1)	0
Other comprehensive income (loss)	(504)	134		(237)	18
Total comprehensive income (loss)	(268)	692		190	975
Cash dividends - ordinary shares	(194)	(235)		(395)	(475)
Dividends attributable to noncontrolling interests	(98)	(89)		(103)	(132)
Stock Repurchased and Retired During Period, Value	(816)			(1,467)
Repurchases of ordinary shares		(533)			(1,000)
Other, including options exercised	33	59		39	66
Ending balance	$ 19,088	$ 21,301	21,407	19,088	21,301
Adoption of ASC 606
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement					(45)
Adoption of ASC 606 | Equity Attributable to Johnson Controls
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement			(45)	0	(45)
Adoption of ASC 606 | Equity Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				0	0
Adoption of ASC 606 | Total Equity Excluding Redeemable Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				0	(45)
Adoption of ASU 2016-16
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement					(546)
Adoption of ASU 2016-16 | Equity Attributable to Johnson Controls
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement			$ (546)	0	(546)
Adoption of ASU 2016-16 | Equity Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				0	0
Adoption of ASU 2016-16 | Total Equity Excluding Redeemable Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				0	(546)
Adoption of ASC 842
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				(5)
Adoption of ASC 842 | Equity Attributable to Johnson Controls
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				(5)	0
Adoption of ASC 842 | Equity Attributable to Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				0	0
Adoption of ASC 842 | Total Equity Excluding Redeemable Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Pronouncement				$ (5)	$ 0