Segment and Related Information - Financial Data for Products and Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 4,379	$ 4,324	$ 8,360	$ 8,496
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	3,748	3,714	7,124	7,256
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	446	448	842	915
Other [Member]
Revenue from External Customer [Line Items]
Electric Utilities' Revenues	$ 185	$ 162	$ 394	$ 325