Segment and Related Information (Financial Data - Inter-segment Billing) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ (3,119)	$ (3,550)	$ (9,521)	$ (10,462)
Electric
Segment Reporting Information [Line Items]
Revenues	(1,520)	(1,521)	(3,945)	(4,002)
Gas
Segment Reporting Information [Line Items]
Revenues	(155)	(164)	(1,043)	(969)
Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	(126)	(122)	(363)	(363)
Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	(406)	(612)	(1,196)	(1,717)
Energy Trading
Segment Reporting Information [Line Items]
Revenues	(1,105)	(1,332)	(3,519)	(3,994)
Reconciliation and Eliminations
Segment Reporting Information [Line Items]
Revenues	195	201	548	585
Reconciliation and Eliminations | Electric
Segment Reporting Information [Line Items]
Revenues	16	13	43	39
Reconciliation and Eliminations | Gas
Segment Reporting Information [Line Items]
Revenues	3	3	8	9
Reconciliation and Eliminations | Gas Storage and Pipelines
Segment Reporting Information [Line Items]
Revenues	11	7	17	29
Reconciliation and Eliminations | Power and Industrial Projects
Segment Reporting Information [Line Items]
Revenues	160	171	462	488
Reconciliation and Eliminations | Energy Trading
Segment Reporting Information [Line Items]
Revenues	4	6	16	18
Reconciliation and Eliminations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 1	$ 2	$ 2