Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 41	$ 51
Interest cost	160	152
Expected return on plan assets	(193)	(198)
Recognized net actuarial loss	97	123
Amortization of prior service cost (benefit)	4	5
Settlement		2
Net periodic benefit cost	109	135
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4
Interest cost	29	28
Recognized net actuarial loss	3	9
Amortization of prior service cost (benefit)	(5)	(4)
Net periodic benefit cost	$ 31	$ 37