Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 32
Interest cost	42	42
Expected return on plan assets	(44)	(48)
Prior service cost (credit) and transition obligation (asset) amortization	(1)	(1)
Actuarial loss (gain) amortization	66	40
Net periodic benefit cost	105	65
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	1	2
Expected return on plan assets		(1)
Actuarial loss (gain) amortization	(3)	(1)
Net periodic benefit cost	$ (1)	$ 1