Other Postretirement Benefits (Net Periodic Postretirement Benefit Costs) (Detail) (Other Postretirement Benefit Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Other Postretirement Benefit Plans, Defined Benefit [Member]
Schedule Of Other Postretirement Benefits [Line Items]
Service cost	$ 6	$ 6	$ 13	$ 12
Interest cost on accumulated other postretirement benefit obligation	18	21	36	42
Expected return on plan assets	(21)	(22)	(42)	(44)
Amortization of net actuarial loss	24	22	49	44
Amortization of prior service cost	(5)	(2)	(11)	(4)
Amortization of transition obligation	1	1	1	2
NET PERIODIC BENEFIT COST	23	26	46	52
Cost capitalized	(8)	(9)	(16)	(18)
Reconciliation to rate level	5	4	14	9
Cost charged to operating expenses	20	21	44	43
Expected contributions	$ 83		$ 83