Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 4,382	$ 4,131
Gas Revenue	1,639	1,560
Retail
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,568	3,394
Wholesale
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	619	531
Other
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	195	206
Gas distribution operations
Revenue from External Customer [Line Items]
Gas Revenue	1,200	1,132
Gas marketing services
Revenue from External Customer [Line Items]
Gas Revenue	271	288
Other
Revenue from External Customer [Line Items]
Gas Revenue	$ 168	$ 140