Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	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	$ 108	$ 217	$ (108)
Expected Rate on Deferred Tax Reversals	21.00%		24.50%
Effective income tax rate	44.00%	128.00%
U.S. federal statutory income tax rate	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
Estimated benefit to tax expense		$ 25
Restructuring and impairment costs	$ 0	154
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	$ (76)
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%