Segment and Related Information (Financial Data - Inter-Segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ (3,271)	$ (3,245)	$ (2,855)	$ (3,236)	$ (2,874)	$ (2,928)	$ (2,262)	$ (2,566)	$ (12,607)	$ (10,630)	$ (10,337)
Reclassifications and Eliminations
Segment Reporting Information [Line Items]
Revenues									704	704	868
Reclassifications and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues									48	43	36
Reclassifications and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues									8	9	3
Reclassifications and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues									42	9	8
Reclassifications and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues									569	602	785
Reclassifications and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues									35	39	32
Reclassifications and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues									$ 2	$ 2	$ 4