Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenue from External Customer [Line Items]
Gas Revenue	$ 543		$ 543
Electric Utilities' Revenues	5,619	$ 5,390	13,979	$ 13,887
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	4,808	4,701	11,932	11,958
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	613	520	1,455	1,435
Other [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	198	$ 169	592	$ 494
Gas Distribution Operations [Member]
Revenue from External Customer [Line Items]
Gas Revenue	420		420
Gas Marketing Services [Member]
Revenue from External Customer [Line Items]
Gas Revenue	126		126
Other Gas Revenue [Member]
Revenue from External Customer [Line Items]
Gas Revenue	$ (3)		$ (3)