Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 51	$ 47
Interest cost	152	160
Expected return on plan assets	(198)	(202)
Recognized net actuarial loss	123	96
Amortization of prior service cost (benefit)	5	5
Settlement	2
Net periodic benefit cost	135	106
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4
Interest cost	28	33
Recognized net actuarial loss	9	6
Amortization of prior service cost (benefit)	(4)	(4)
Net periodic benefit cost	$ 37	$ 39