Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 288	$ 190	$ 155
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	186	165	148
Interest cost	639	678	682
Expected return on plan assets	(808)	(806)	(787)
Recognized net actuarial loss	384	247	181
Amortization of prior service cost (benefit)	19	19	17
Settlements		2	2
Curtailments		(9)	(10)
Net periodic benefit cost	420	296	233
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	17	20
Interest cost	131	158	174
Expected return on plan assets		(2)	(7)
Recognized net actuarial loss	25	26	33
Amortization of prior service cost (benefit)	(16)	(17)	(15)
Settlements			(3)
Net periodic benefit cost	$ 154	$ 182	$ 202