Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 33	$ 25
Interest cost	47	46
Expected return on plan assets	(40)	(35)
Amortization of unrecognized (gain) loss and prior service cost	18	5
Net periodic benefit costs	58	41
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	13
Interest cost	28	31
Expected return on plan assets	(1)	(1)
Amortization of unrecognized (gain) loss and prior service cost	3	(1)
Net periodic benefit costs	$ 44	$ 42