Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ (3,753)	$ (3,236)
Electric
Segment Reporting Information [Line Items]
Revenues	(1,205)
Gas
Segment Reporting Information [Line Items]
Revenues	(550)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	(119)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	(567)
Energy Trading
Segment Reporting Information [Line Items]
Revenues	(1,498)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	186	202
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	13	12
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	2	3
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	8	7
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	155	168
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	7	11
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 1