Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	$ 2,387	$ 2,190	$ 7,128	$ 6,442
Operating Segments [Member] | Electric [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1,458	1,543	3,942	4,031
Operating Segments [Member] | Gas [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	161	160	1,013	883
Operating Segments [Member] | Gas Storage and Pipelines [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	34	21	92	73
Operating Segments [Member] | Power and Industrial Projects [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	512	477	1,431	1,375
Operating Segments [Member] | Energy Trading [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	463	223	1,307	723
Operating Segments [Member] | Corporate and Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1	0	2	2
Intersegment Eliminations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	242	234	659	645
Intersegment Eliminations [Member] | Electric [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	6	7	19	22
Intersegment Eliminations [Member] | Gas [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	2	1	4	3
Intersegment Eliminations [Member] | Gas Storage and Pipelines [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	(2)	1	2	5
Intersegment Eliminations [Member] | Power and Industrial Projects [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	231	225	621	608
Intersegment Eliminations [Member] | Energy Trading [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	13	10	36	34
Intersegment Eliminations [Member] | Corporate and Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	(8)	(11)	(23)	(28)
Intersegment Eliminations [Member] | Discontinued Operations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Inter-segment Revenues	$ 0	$ 1	$ 0	$ 1