Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,550	$ 3,245	$ 10,462	$ 9,336
Electric
Disaggregation of Revenue [Line Items]
Revenues	1,521		4,002
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	6		15
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues	773		1,943
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues	495		1,370
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues	173		527
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	80		162
Gas
Disaggregation of Revenue [Line Items]
Revenues	164		969
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	2		6
Gas | Alternative Revenue Programs
Disaggregation of Revenue [Line Items]
Revenues			(4)
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	32		37
Gas | Gas sales
Disaggregation of Revenue [Line Items]
Revenues	89		726
Gas | End User Transportation
Disaggregation of Revenue [Line Items]
Revenues	34		168
Gas | Intermediate Transportation
Disaggregation of Revenue [Line Items]
Revenues	9		38
Gas Storage and Pipelines
Disaggregation of Revenue [Line Items]
Revenues	122		363
Power and Industrial Projects
Disaggregation of Revenue [Line Items]
Revenues	612		1,717
Lease revenue outside scope of Topic 606	37		93
Amount previously disclosed as lease revenues reclassificed to be within scope of Topic 606	788
Energy Trading
Disaggregation of Revenue [Line Items]
Revenues	1,332		3,994
Gain (loss) on derivative outside scope of Topic 606	$ 1,100		$ 3,200