Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ (3,159)	$ (2,855)	$ (6,912)	$ (6,091)
Electric
Segment Reporting Information [Line Items]
Revenues	(1,276)		(2,481)
Gas
Segment Reporting Information [Line Items]
Revenues	(255)		(805)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	(122)		(241)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	(538)		(1,105)
Energy Trading
Segment Reporting Information [Line Items]
Revenues	(1,164)		(2,662)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	198	194	384	396
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	13	13	26	25
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	4	2	6	5
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	14	15	22	22
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	162	156	317	324
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	5	8	12	19
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 0	$ 0	$ 1	$ 1