Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ (3,245)	$ (2,928)	$ (9,336)	$ (7,756)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	169	210	565	553
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	11	15	36	32
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	1	5	6	8
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	10	2	32	7
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	138	178	462	476
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	8	10	27	28
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 0	$ 2	$ 2