Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 33	$ 126	$ 97
Interest cost	43	42	128	126
Expected return on plan assets	(44)	(48)	(132)	(143)
Prior service cost (credit) and transition obligation (asset) amortization	(1)	(2)	(4)	(4)
Actuarial loss (gain) amortization	66	41	198	121
Net periodic benefit cost	106	66	316	197
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	2	3	4
Interest cost	1	1	3	5
Expected return on plan assets		(1)	(1)	(2)
Prior service cost (credit) and transition obligation (asset) amortization
Actuarial loss (gain) amortization	(3)	(1)	(8)	(5)
Net periodic benefit cost	$ (1)	$ 1	$ (3)	$ 2