Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Mar. 31, 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
Effective income tax rate	14.00%		129.00%		31.00%	62.00%
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	$ 99	$ 78	$ 158	$ 177
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount			8	46
Net actuarial gain	0		18	117	$ 0	$ 135
Transaction and integration costs	64	50	138	130
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Amount		$ 24	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%
Change in Entity Status [Domain]
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount				101
Transaction and integration costs [Member]
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	$ 9	$ 7	31	$ 11
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
Other Property [Member] | Blended 2018 U.S. Effective Tax Rate [Member]
Income Taxes, Additional Information [Line Items]
U.S. federal statutory income tax rate		8.00%
Cash [Member] | Blended 2018 U.S. Effective Tax Rate [Member]
Income Taxes, Additional Information [Line Items]
U.S. federal statutory income tax rate		15.50%
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%	12.50%