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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Remeasurement gain, net	$ (237)		$ (237)
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	98	90	197	179
Amortization of prior service cost (credit)	2	(1)	3	(2)
Subtotal	100	89	200	177
Expected return on plan assets	(311)	(443)	(622)	(885)
Interest cost	251	362	501	724
Subtotal	40	8	79	16
Remeasurement gain, net	(237)		(237)
Net periodic benefit (income) cost	(197)	8	(158)	16
Health Care And Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	79	93	159	185
Amortization of prior service cost (credit)	(61)	(9)	(123)	(18)
Subtotal	18	84	36	167
Expected return on plan assets	(36)	(42)	(72)	(85)
Interest cost	274	326	548	653
Subtotal	256	368	512	735
Net periodic benefit (income) cost	$ 256	$ 368	$ 512	$ 735