Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ (3,514)	$ (3,753)
Electric
Segment Reporting Information [Line Items]
Revenues	(1,235)	(1,205)
Gas
Segment Reporting Information [Line Items]
Revenues	(645)	(550)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	(116)	(119)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	(388)	(567)
Energy Trading
Segment Reporting Information [Line Items]
Revenues	(1,301)	(1,498)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	172	186
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	14	13
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	2	2
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	3	8
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	145	155
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	7	7
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 1