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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 48	$ 83	$ 99
Interest cost	159	151	319	303
Expected return on plan assets	(195)	(198)	(388)	(396)
Recognized net actuarial loss	98	124	195	247
Amortization of prior service cost (benefit)	5	5	9	10
Settlement				2
Net periodic benefit cost	109	130	218	265
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	8	9
Interest cost	29	29	58	57
Recognized net actuarial loss	3	8	6	17
Amortization of prior service cost (benefit)	(4)	(5)	(9)	(9)
Net periodic benefit cost	$ 32	$ 37	$ 63	$ 74