Segment and Related Information - Electric Utilities' Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Electric Utilities Revenues	$ 5,181	$ 6,264	$ 4,459	$ 3,992	$ 3,568	$ 5,401	$ 4,337	$ 4,183	$ 19,896	$ 17,489	$ 18,467
Retail [Member]
Revenue from External Customer [Line Items]
Electric Utilities Revenues									15,234	14,987	15,550
Wholesale [Member]
Revenue from External Customer [Line Items]
Electric Utilities Revenues									1,926	1,798	2,184
Other Electric Revenue [Member]
Revenue from External Customer [Line Items]
Electric Utilities Revenues									781	657	672
Electric Utilities [Member]
Revenue from External Customer [Line Items]
Electric Utilities Revenues									$ 17,941	$ 17,442	$ 18,406