Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 33	$ 30	$ 97	$ 89
Interest cost	42	42	126	126
Expected return on plan assets	(48)	(52)	(143)	(155)
Prior service cost (credit) and transition obligation (asset) amortization	(2)	(2)	(4)	(7)
Actuarial loss (gain) amortization	41	31	121	94
Net periodic benefit cost	66	49	197	147
Postretirement Welfare Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1	4	3
Interest cost	1	3	5	7
Expected return on plan assets	(1)	(1)	(2)	(3)
Actuarial loss (gain) amortization	(1)	(2)	(5)	(5)
Net periodic benefit cost	$ 1	$ 1	$ 2	$ 2