Segment and Related Information - Electric Utilities' Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financial data for Products and Services
Electric Utilities Revenues	$ 3,568	$ 5,401	$ 4,337	$ 4,183	$ 4,017	$ 5,339	$ 4,467	$ 4,644	$ 17,489	$ 18,467	$ 17,087
Retail [Member]
Financial data for Products and Services
Electric Utilities Revenues									14,987	15,550	14,541
Wholesale [Member]
Financial data for Products and Services
Electric Utilities Revenues									1,798	2,184	1,855
Other Electric Revenue [Member]
Financial data for Products and Services
Electric Utilities Revenues									657	672	639
Electric Utilities [Member]
Financial data for Products and Services
Electric Utilities Revenues									$ 17,442	$ 18,406	$ 17,035