Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 5,523	$ 5,619	$ 14,239	$ 13,979
Gas Revenue	565	543	2,841	543
Retail
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	4,615	4,808	11,786	11,932
Wholesale
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	718	613	1,867	1,455
Other
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	190	198	586	592
Gas distribution operations
Revenue from External Customer [Line Items]
Gas Revenue	430	420	2,119	420
Gas marketing services
Revenue from External Customer [Line Items]
Gas Revenue	143	126	597	126
Other
Revenue from External Customer [Line Items]
Gas Revenue	$ (8)	$ (3)	$ 125	$ (3)