Pension and Other Postretirement 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 Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 48	$ 46	$ 99	$ 93
Interest cost	151	160	303	320
Expected return on plan assets	(198)	(201)	(396)	(403)
Recognized net actuarial loss	124	95	247	191
Amortization of prior service cost(benefit)	5	4	10	9
Settlement			2
Net periodic benefit cost	130	104	265	210
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	3	9	7
Interest cost	29	33	57	66
Recognized net actuarial loss	8	6	17	12
Amortization of prior service cost(benefit)	(5)	(4)	(9)	(8)
Net periodic benefit cost	$ 37	$ 38	$ 74	$ 77