Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 5,390	$ 5,327	$ 13,887	$ 14,408
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	4,701	4,558	11,958	12,186
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	520	600	1,435	1,719
Other [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 169	$ 169	$ 494	$ 503