Segment and Related Information - Electric Utilities' Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Total operating revenues	$ 23,495	$ 23,031	$ 19,896
Retail electric revenues
Revenue from External Customer [Line Items]
Total operating revenues	15,222	15,330	15,234
Other electric revenues
Revenue from External Customer [Line Items]
Total operating revenues	664	681	698
Electric Utilities
Revenue from External Customer [Line Items]
Total operating revenues	18,571	18,540	17,941
Electric Utilities | Retail electric revenues
Revenue from External Customer [Line Items]
Total operating revenues	15,222	15,330	15,234
Electric Utilities | Wholesale
Revenue from External Customer [Line Items]
Total operating revenues	2,516	2,426	1,926
Electric Utilities | Other electric revenues
Revenue from External Customer [Line Items]
Total operating revenues	833	784	781
Electric Utilities | Electric Utilities
Revenue from External Customer [Line Items]
Total operating revenues	$ 18,571	$ 18,540	$ 17,941