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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Remeasurement (gain) loss, net			$ (237)
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	296	268	99	89
Amortization of prior service cost (credit)	5	(3)	2	(1)
Subtotal	301	265	101	88
Expected return on plan assets	(933)	(1,327)	(311)	(442)
Interest cost	752	1,086	251	362
Subtotal	120	24
Remeasurement (gain) loss, net	(237)
Net periodic benefit (income) cost	(117)	24	41	8
Health Care And Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	238	277	79	92
Amortization of prior service cost (credit)	(185)	(26)	(62)	(8)
Subtotal	53	251	17	84
Expected return on plan assets	(108)	(129)	(36)	(44)
Interest cost	821	980	273	327
Subtotal	766	1,102
Net periodic benefit (income) cost	$ 766	$ 1,102	$ 254	$ 367