Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ (3,550)	$ (3,245)	$ (10,462)	$ (9,336)
Electric
Segment Reporting Information [Line Items]
Revenues	(1,521)		(4,002)
Gas
Segment Reporting Information [Line Items]
Revenues	(164)		(969)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	(122)		(363)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	(612)		(1,717)
Energy Trading
Segment Reporting Information [Line Items]
Revenues	(1,332)		(3,994)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	201	169	585	565
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	13	11	39	36
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	3	1	9	6
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	7	10	29	32
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	171	138	488	462
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	6	8	18	27
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 1	$ 2	$ 2