Benefit Plans (Components of Net Periodic Benefit Cost) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 6	$ 17	$ 18
Interest cost	39	41	114	123
Expected return on plan assets	(47)	(47)	(140)	(141)
Amortization of unrecognized net loss	11	8	34	22
Net periodic benefit cost	8	8	25	22
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	1	1
Interest cost	1	1	4	5
Expected return on plan assets	(1)		(3)	(2)
Amortization of unrecognized net loss		1		1
Amortization of unrecognized prior service benefit	(6)	(7)	(19)	(20)
Regulatory asset decrease		1		4
Net periodic benefit cost	$ (5)	$ (3)	$ (17)	$ (11)