Property, Plant and Equipment (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Property, Plant and Equipment [Line Items]
Generation	$ 10,383	$ 9,785
Distribution	7,306	7,003
Total	17,689	16,788
Generation	(3,880)	(3,946)
Distribution	(2,837)	(2,580)
Total	(6,717)	(6,526)
Net Property, Plant and Equipment	$ 10,972	$ 10,262