Guarantees - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Guarantor Obligations [Line Items]
Post sale contingent tax indemnification liabilities	$ 276	$ 290	$ 290
Maximum length, in years, of a product warranty for it to be recorded in other current liabilities	1 year
Minimum length, in years, of a product warranty for it to be recorded in other noncurrent liabilities	1 year
Business Divestitures, Not Specific [Member]
Guarantor Obligations [Line Items]
Post sale contingent tax indemnification liabilities	$ 235