Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Settlement gain	$ (1)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	29		25	94	75
Interest cost	48		45	142	137
Expected return on plan assets	(41)		(33)	(121)	(103)
Amortization of unrecognized (gain) loss and prior service cost	8		4	41	15
Curtailment loss		2		2
Settlement gain	(1)	(1)		(2)
Total	43		41	156	124
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	14		11	42	37
Interest cost	29		30	85	93
Expected return on plan assets	(1)			(2)	(2)
Amortization of unrecognized (gain) loss and prior service cost	2			7	(2)
Total	$ 44		$ 41	$ 132	$ 126