Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Amortization of Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ 2	$ 1	$ 3	$ 2
Amortization of Prior Service Credit [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1)	(1)	(1)	(2)
Settlement and Curtailment (Credit) Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax				(1)
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	1		2	(1)
Tax impact		1		$ 1
Total after tax	$ 1	$ 1	$ 2