Segment and Related Information (Financial Data - Inter-Segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ (3,148)	$ (3,119)	$ (2,888)	$ (3,514)	$ (3,750)	$ (3,550)	$ (3,159)	$ (3,753)	$ (12,669)	$ (14,212)	$ (12,607)
Electric
Segment Reporting Information [Line Items]
Revenues									(5,229)	(5,298)
Gas
Segment Reporting Information [Line Items]
Revenues									(1,482)	(1,436)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues									(501)	(485)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues									(1,560)	(2,204)
Energy Trading
Segment Reporting Information [Line Items]
Revenues									(4,610)	(5,557)
Reclassifications and Eliminations
Segment Reporting Information [Line Items]
Revenues									715	771	704
Reclassifications and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues									56	52	48
Reclassifications and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues									12	12	8
Reclassifications and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues									27	36	42
Reclassifications and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues									596	642	569
Reclassifications and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues									22	27	35
Reclassifications and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues									$ 2	$ 2	$ 2