Segment and Related Information (Financial Data - Inter-Segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ (3,750)	$ (3,550)	$ (3,159)	$ (3,753)	$ (3,271)	$ (3,245)	$ (2,855)	$ (3,236)	$ (14,212)	$ (12,607)	$ (10,630)
Electric
Segment Reporting Information [Line Items]
Revenues									(5,298)
Gas
Segment Reporting Information [Line Items]
Revenues									(1,436)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues									(485)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues									(2,204)
Energy Trading
Segment Reporting Information [Line Items]
Revenues									(5,557)
Reclassifications and Eliminations
Segment Reporting Information [Line Items]
Revenues									771	704	704
Reclassifications and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues									52	48	43
Reclassifications and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues									12	8	9
Reclassifications and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues									36	42	9
Reclassifications and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues									642	569	602
Reclassifications and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues									27	35	39
Reclassifications and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues									$ 2	$ 2	$ 2