Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2016
Income Taxes, Additional Information [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount							$ 108
Income tax provision	$ 47		$ 36		$ 155	$ 253	$ (108)
Expected Rate on Deferred Tax Reversals	21.00%						24.50%
Effective income tax rate	15.00%		14.00%		27.00%	60.00%
U.S. federal statutory income tax rate	12.50%				12.50%	12.50%
Gross tax effected unrecognized tax benefits							$ 2,358
Amount of unrecognized tax benefits which may impact effective tax rate							2,225
Total net accrued interest, net of tax benefit							$ 119
Income Tax Examination, Penalties and Interest Accrued	$ 31				$ 31
Estimated benefit to tax expense				$ 25
Restructuring and impairment costs	$ 0		$ 0	154	0	$ 154
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		$ (76)
Net actuarial gain					$ (1)	$ 0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Amount				(23)
2016 Restructuring Plan [Member]
Income Taxes, Additional Information [Line Items]
Restructuring and impairment costs								$ 620
2016 Restructuring Plan [Domain]
Income Taxes, Additional Information [Line Items]
Restructuring and impairment costs								$ 222
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%