Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2016
Goodwill And Intangible Assets [Line Items]
Goodwill	$ 151		$ 151		$ 152
Amortization expense estimated for year one	7		7
Amortization expense estimated for year two	7		7
Amortization expense estimated for year three	7		7
Amortization expense estimated for year four	7		7
Amortization expense estimated for year five	7		7
Cost of Sales and Selling and Administrative Expenses [Member]
Goodwill And Intangible Assets [Line Items]
Amortization of intangible assets	$ 2	$ 2	$ 4	$ 4
Minimum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			14 years
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			25 years
Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Useful life of intangible assets			19 years