Equity and Noncontrolling Interests Equity Attributable to Johnson Controls and Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Equity [Line Items]
Deferred tax asset from stock options exercised and restricted stock vestings	$ 179
Stock repurchase program, authorized amount	1,000		$ 1,000
Payments for Repurchase of Common Stock			300	$ 651	$ 501
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	20,447	$ 24,118	20,447	24,118	10,335
Beginning balance	920		920
Net income (loss)			2,162	1,611	(868)
Foreign currency translation adjustments			(483)	103	(94)
Realized and unrealized gains (losses) on marketable securities			4	5	(1)
Pension and postretirement plans			0	0	(1)
Other comprehensive income (loss)			(508)	94	(87)
Comprehensive income (loss) attributable to Johnson Controls			1,689	1,710	(964)
Comprehensive income attributable to noncontrolling interest			186	203	225
Total comprehensive income (loss)			1,875	1,913	(739)
Result of contribution of Johnson Controls, Inc. to Johnson Controls International plc					15,808
Cash dividends - common stock			(968)	(938)	(752)
Repurchases of common stock			300	651	501
Cumulative Effect on Retained Earnings, Net of Tax			(179)
Spin-off of Adient				4,038
Other, including options exercised			(117)	(246)	(192)
Ending balance			21,164	20,447	24,118
Ending balance			1,294	920
Ending balance			22,458	21,367
Beginning balance	21,367		21,367
Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	20,447	24,118	20,447	24,118	10,335
Net income (loss)			2,162	1,611	(868)
Foreign currency translation adjustments			(458)	108	(105)
Realized and unrealized gains (losses) on derivatives			(19)	(14)	11
Realized and unrealized gains (losses) on marketable securities			4	5	(1)
Pension and postretirement plans					(1)
Other comprehensive income (loss)			(473)	99	(96)
Comprehensive income (loss) attributable to Johnson Controls			1,689	1,710	(964)
Result of contribution of Johnson Controls, Inc. to Johnson Controls International plc				(4,038)	(15,808)
Cash dividends - common stock			(968)	(938)	(752)
Dividends attributable to noncontrolling interests			0	0	0
Repurchases of common stock			(300)	(651)	(501)
Change in noncontrolling interest share			0	0	0
Cumulative Effect on Retained Earnings, Net of Tax			179
Noncontrolling Interest, Change in Redemption Value			0
Spin-off of Adient		(4,000)
Other, including options exercised			117	246	192
Ending balance			21,164	20,447	24,118
Nonredeemable Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	920	972	920	972	163
Income from continuing operations attributable to noncontrolling interests			186	164	168
Foreign currency translation adjustments			(22)	(18)	9
Realized and unrealized gains (losses) on derivatives			(1)	1	(1)
Realized and unrealized gains (losses) on marketable securities			0	0	0
Realized and unrealized gains on marketable securities					0
Other comprehensive income (loss)			(23)	(17)	8
Comprehensive income attributable to noncontrolling interest			163	147	176
Result of contribution of Johnson Controls, Inc. to Johnson Controls International plc				(138)	0
Cash dividends - common stock			0	0	0
Dividends attributable to noncontrolling interests			(43)	(56)	(93)
Repurchases of common stock			0	0	0
Change in noncontrolling interest share			23	(5)	726
Cumulative Effect on Retained Earnings, Net of Tax			0
Noncontrolling Interest, Change in Redemption Value			231
Other, including options exercised			0	0	0
Ending balance			1,294	920	972
Stockholders' Equity, Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (loss)			2,348	1,775	(700)
Foreign currency translation adjustments			(480)	90	(96)
Realized and unrealized gains (losses) on derivatives			(20)	(13)	10
Realized and unrealized gains (losses) on marketable securities			4	5	(1)
Pension and postretirement plans					(1)
Other comprehensive income (loss)			(496)	82	(88)
Total comprehensive income (loss)			1,852	1,857	(788)
Result of contribution of Johnson Controls, Inc. to Johnson Controls International plc				(4,176)	(15,808)
Cash dividends - common stock			(968)	(938)	(752)
Dividends attributable to noncontrolling interests			(43)	(56)	(93)
Repurchases of common stock			(300)	(651)	(501)
Change in noncontrolling interest share			23	(5)	726
Cumulative Effect on Retained Earnings, Net of Tax			179
Noncontrolling Interest, Change in Redemption Value			231
Other, including options exercised			117	246	192
Ending balance			22,458	21,367	25,090
Beginning balance	$ 21,367	$ 25,090	$ 21,367	$ 25,090	$ 10,498