Segment and Related Information (Financial Data - Inter-Segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating Revenues	$ (3,288)	$ (3,284)	$ (2,583)	$ (3,022)	$ (3,148)	$ (3,119)	$ (2,888)	$ (3,514)	$ (12,177)	$ (12,669)	$ (14,212)
Operating Revenues — Non-utility operations									(5,332)	(6,031)	(7,542)
Electric
Segment Reporting Information [Line Items]
Operating Revenues									(5,520)	(5,229)	(5,298)
Gas
Segment Reporting Information [Line Items]
Operating Revenues									(1,414)	(1,482)	(1,436)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Operating Revenues									(754)	(501)	(485)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Operating Revenues									(1,224)	(1,560)	(2,204)
Energy Trading
Segment Reporting Information [Line Items]
Operating Revenues									(3,863)	(4,610)	(5,557)
Reclassifications and Eliminations
Segment Reporting Information [Line Items]
Operating Revenues									600	715	771
Operating Revenues — Non-utility operations									525	647	707
Reclassifications and Eliminations | Electric
Segment Reporting Information [Line Items]
Operating Revenues									61	56	52
Operating Revenues — Non-utility operations									2
Reclassifications and Eliminations | Gas
Segment Reporting Information [Line Items]
Operating Revenues									16	12	12
Reclassifications and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Operating Revenues									26	27	36
Reclassifications and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Operating Revenues									464	596	642
Reclassifications and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Operating Revenues									31	22	27
Reclassifications and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Operating Revenues									$ 2	$ 2	$ 2