Benefit or Income Cost Related to Pension and Postretirement Health Care and Life Insurance Plans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 99	$ 89
Amortization of prior service cost (credit)	1	(1)
Subtotal	100	88
Expected return on plan assets	(311)	(442)
Interest cost	250	362
Net periodic benefit cost	39	8
Health Care And Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	92
Amortization of prior service cost (credit)	(62)	(9)
Subtotal	18	83
Expected return on plan assets	(36)	(43)
Interest cost	274	327
Net periodic benefit cost	$ 256	$ 367