Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 32	$ 84	$ 64
Interest cost	43	42	85	84
Expected return on plan assets	(44)	(47)	(88)	(95)
Prior service cost (credit) and transition obligation (asset) amortization	(2)	(1)	(3)	(2)
Actuarial loss (gain) amortization	66	40	132	80
Net periodic benefit cost	105	66	210	131
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	1	2	2	4
Expected return on plan assets	(1)		(1)	(1)
Prior service cost (credit) and transition obligation (asset) amortization
Actuarial loss (gain) amortization	(2)	(3)	(5)	(4)
Net periodic benefit cost	$ (1)		$ (2)	$ 1