Accumulated Other Comprehensive Loss - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax			$ 1	$ (1)	$ 28
Tax impact	$ 1			1	(6)
Total after tax	1	$ (1)	1		22
Pension and Other Postretirement Benefit Liability Adjustments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of actuarial losses	1		$ 1	2	2
Amortization of prior service cost	(1)			(2)	(1)
Settlement and curtailment (credit) cost				$ 1	$ (27)
Total after tax	$ 1	$ (1)