Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Amortization of Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ 1	$ 1
Amortization of Prior Service Credit [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax		(1)
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	(1)
Total after tax	$ 1	$ (1)