Property, Plant and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment, Net, by Type [Abstract]
Total	$ 11,496	$ 10,840
Accumulated depreciation	3,633	3,443
Property, plant and equipment, net	7,863	7,397
Depreciation, Depletion and Amortization [Abstract]
Depreciation expense	354	349	$ 316
Amortization of securitized regulatory assets	329	455	365
Other amortization	41	34	24
Total depreciation and amortization	724	838	705
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	33	37
Accretion expense	1	1
Revisions in estimates of cash flows	1	(5)
Ending balance	35	33	$ 37
Transmission [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	2,767	2,402
Distribution [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	7,178	6,965
Other [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Total	$ 1,551	$ 1,473
Weighted Average [Member] | Transmission [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life	45 years
Weighted Average [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life	30 years
Weighted Average [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life	15 years