Segment and Related Information - Electric Utilities' Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Electric utilities revenues	$ 5,629	$ 6,201	$ 5,430	$ 5,771	$ 5,181	$ 6,264	$ 4,459	$ 3,992	$ 23,031	$ 19,896	$ 17,489
Retail
Revenue from External Customer [Line Items]
Electric utilities revenues									15,330	15,234	14,987
Wholesale
Revenue from External Customer [Line Items]
Electric utilities revenues									2,426	1,926	1,798
Other Electric Revenue [Member]
Revenue from External Customer [Line Items]
Electric utilities revenues									784	781	657
Electric Utilities
Revenue from External Customer [Line Items]
Electric utilities revenues									$ 18,540	$ 17,941	$ 17,442