Segment and Related Information (Financial Data - Operating Revenues including inter segment revenues) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	$ 2,598	$ 2,595	$ 7,850	$ 9,223
Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(223)	(230)	(660)	(655)
Electric | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1,386	1,358	3,737	4,049
Electric | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(9)	(8)	(27)	(22)
Gas | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	153	149	1,018	1,182
Gas | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(1)	(1)	(2)	(5)
Gas Storage and Pipelines | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	61	49	175	146
Gas Storage and Pipelines | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(1)	(1)	(7)	(9)
Power and Industrial Projects | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	585	605	1,708	1,703
Power and Industrial Projects | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(204)	(211)	(599)	(593)
Energy Trading | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	635	664	1,870	2,797
Energy Trading | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(7)	(8)	(23)	(24)
Corporate and Other | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1	0	2	1
Corporate and Other | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	$ (1)	$ (1)	$ (2)	$ (2)