Property, Plant and Equipment (Summary of property by classification) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	$ 12,589	$ 11,779
Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment - Generation	11,641	11,127
Less Accumulated Depreciation and Amortization - Total	(4,149)	(4,004)
Electric Transmission and Distribution Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment - Distribution	8,164	7,603
Less Accumulated Depreciation and Amortization - Total	(3,067)	(2,947)
Electric Generation, Transmission and Distribution Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment - Total	19,805	18,730
Less Accumulated Depreciation and Amortization - Total	$ (7,216)	$ (6,951)