Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 9	$ 22	$ 19
Interest cost	9	10	18	20
Expected return on assets	(15)	(13)	(31)	(27)
Amortization of:
Prior service credit	(3)	(3)	(5)	(5)
Actuarial loss	7	8	14	16
Total net periodic benefit cost	10	11	18	23
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost
Interest cost		1		1
Expected return on assets
Amortization of:
Prior service credit		(1)	(1)	(2)
Actuarial loss
Total net periodic benefit cost			$ (1)	$ (1)