Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 4,131	$ 3,981
Gas Revenue	1,560
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,394	3,377
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	531	396
Other [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	206	$ 208
Gas Distribution Operations [Member]
Revenue from External Customer [Line Items]
Gas Revenue	1,132
Gas Marketing Services [Member]
Revenue from External Customer [Line Items]
Gas Revenue	288
Other Gas Revenue [Member]
Revenue from External Customer [Line Items]
Gas Revenue	$ 140