Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 4,585	$ 4,379	$ 8,716	$ 8,360
Gas Revenue	716		2,276
Retail
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,777	3,748	7,171	7,124
Wholesale
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	618	446	1,149	842
Other
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	190	$ 185	396	$ 394
Gas distribution operations
Revenue from External Customer [Line Items]
Gas Revenue	557		1,689
Gas marketing services
Revenue from External Customer [Line Items]
Gas Revenue	166		454
Other
Revenue from External Customer [Line Items]
Gas Revenue	$ (7)		$ 133