Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 3,954	$ 4,172
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,377	3,542
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	396	467
Other [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 181	$ 163