Property, Plant and Equipment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment, Net, by Type [Abstract]
Total	$ 8,204	$ 7,827
Accumulated depreciation	2,839	2,784
Property, plant and equipment, net	5,365	5,043
Depreciation, Depletion and Amortization [Abstract]
Depreciation expense	265	257	273
Amortization of securitized regulatory assets	442	308	289
Other amortization	22	22	20
Total depreciation and amortization	729	587	582
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balances	23	26
Accretion expense	1	1
Revision in estimate of cash flows	5	(4)
Ending balance	29	23	26
Increase (decrease) in revisions in estimates of cash flows	5	4
Transmission [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	1,851	1,720
Property, plant and equipment, useful life, weighted average (in years)	41
Distribution [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	5,548	5,327
Property, plant and equipment, useful life, weighted average (in years)	29
Other [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	$ 805	$ 780
Property, plant and equipment, useful life, weighted average (in years)	15