Segment and Related Information (Financial Data - Operating Revenues including Inter-segment Revenues) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Utility operations	$ 1,514	$ 1,423	$ 3,254	$ 3,141
Operating Revenues — Non-utility operations	1,645	1,432	3,658	2,950
Operating Revenues	3,159	2,855	6,912	6,091
Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Non-utility operations	(198)	(194)	(384)	(396)
Operating Revenues	(198)	(194)	(384)	(396)
Electric
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1,276		2,481
Electric | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Utility operations	1,276	1,218	2,481	2,393
Electric | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(13)	(13)	(26)	(25)
Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	255		805
Gas | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Utility operations	255	220	805	777
Gas | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(4)	(2)	(6)	(5)
Gas Storage and Pipelines
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	122		241
Gas Storage and Pipelines | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Non-utility operations	122	113	241	218
Gas Storage and Pipelines | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(14)	(15)	(22)	(22)
Power and Industrial Projects
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	538		1,105
Power and Industrial Projects | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Non-utility operations	538	507	1,105	1,055
Power and Industrial Projects | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(162)	(156)	(317)	(324)
Energy Trading
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	1,164		2,662
Energy Trading | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Non-utility operations	1,164	991	2,662	2,043
Energy Trading | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	(5)	(8)	(12)	(19)
Corporate and Other | Operating segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues — Non-utility operations	2	0	2	1
Corporate and Other | Reconciliation and Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating Revenues	$ 0	$ 0	$ (1)	$ (1)