Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 9	$ 11	$ 19	$ 20
Interest cost	10	9	20	19
Expected return on assets	(13)	(13)	(27)	(27)
Amortization of:
Prior service credit	(3)	(2)	(5)	(4)
Actuarial loss	8	4	16	9
Total net periodic benefit cost	11	9	23	17
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost
Interest cost	1	1	1	1
Expected return on assets
Amortization of:
Prior service credit	(1)	(1)	(2)	(2)
Actuarial loss
Total net periodic benefit cost			$ (1)	$ (1)