Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Income Taxes, Additional Information [Line Items]
Discrete non-cash tax charge			$ 101
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount			$ 305
Transition tax percentage cash assets			15.50%
Transition tax percentage, other property			8.00%
Effective income tax rate							22.00%	38.00%
U.S. federal statutory income tax rate			21.00%					35.00%
Gross tax effected unrecognized tax benefits									$ 2,173
Amount of unrecognized tax benefits which may impact effective tax rate									2,047
Total net accrued interest, net of tax benefit									$ 99
Estimated benefit to tax expense			$ 25
Restructuring and impairment costs	$ 0		158	$ 49	$ 99	$ 78	$ 158	$ 226
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount				(18)	8	46
Net actuarial gain	0			(45)	18	117	$ 0	$ 90
Transaction and integration costs	51	$ 64	50	70	138	130
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Amount			$ (24)	15	20	14
Blended 2018 U.S. Effective Tax Rate [Member]
Income Taxes, Additional Information [Line Items]
U.S. federal statutory income tax rate			24.50%
Transaction and integration costs [Member]
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	$ (6)	$ (9)	$ (7)	$ (11)	(31)	(11)
Change in Entity Status
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount						$ (101)
Outside basis difference of certain subsidiaries of the Scott Safety business [Domain]
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount					$ (457)
Scott Safety business [Member]
Income Taxes, Additional Information [Line Items]
Gain on sale of Scott Safety business			114
Tax effect of gain on sale of Scott Safety business			$ 30
IRELAND
Income Taxes, Additional Information [Line Items]
U.S. federal statutory income tax rate	12.50%						12.50%	12.50%