Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2019	Sep. 30, 2018
Loss Contingencies [Line Items]
Estimated asbestos related net liability on a discounted basis	$ 175	$ 173
Reserves for environmental liabilities	30	37
Conditional asset retirement obligations	29	29
Accrued Environmental Loss Contingencies, Current	15	10
Accrued Environmental Loss Contingencies, Noncurrent	15	27
Liability for Asbestos and Environmental Claims, Gross	541	550
Restricted Cash and Investments	366	377
Restricted Cash and Cash Equivalents	19	15
Insurance Recoveries	84	90
Insurable liabilities	423	417
Insurance Settlements Receivable, Current	6	6
Insurance Settlements Receivable	26	26
Insurance Settlements Receivable, Noncurrent	20	20
Asbestos Issue [Member]
Loss Contingencies [Line Items]
Restricted Cash and Cash Equivalents	19	6
Restricted Investments	263	281
Accrued Compensation and Benefits [Member]
Loss Contingencies [Line Items]
Insurable liabilities	22	22
Other Current Assets
Loss Contingencies [Line Items]
Restricted Cash and Investments	51	33
Other Noncurrent Assets
Loss Contingencies [Line Items]
Restricted Cash and Investments	315	344
Other Noncurrent Liabilities [Member]
Loss Contingencies [Line Items]
Liability for Asbestos and Environmental Claims, Gross	486	495
Insurable liabilities	306	300
Other Current Liabilities
Loss Contingencies [Line Items]
Liability for Asbestos and Environmental Claims, Gross	55	55
Insurable liabilities	$ 95	$ 95