Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 3,753	$ 3,236
Electric
Disaggregation of Revenue [Line Items]
Revenues	1,205
Reduction of revenue due to Tax Cuts and Jobs Act	39
Other revenue outside scope of Topic 606	5
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues	586
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues	429
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues	176
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	14
Gas
Disaggregation of Revenue [Line Items]
Revenues	550
Reduction of revenue due to Tax Cuts and Jobs Act	32
Other revenue outside scope of Topic 606	2
Alternative revenue programs	(3)
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	(10)
Gas | Gas sales
Disaggregation of Revenue [Line Items]
Revenues	457
Gas | End User Transportation
Disaggregation of Revenue [Line Items]
Revenues	85
Gas | Intermediate Transportation
Disaggregation of Revenue [Line Items]
Revenues	18
Gas Storage and Pipelines
Disaggregation of Revenue [Line Items]
Revenues	119
Power and Industrial Projects
Disaggregation of Revenue [Line Items]
Revenues	567
Lease revenue outside scope of Topic 606	445
Energy Trading
Disaggregation of Revenue [Line Items]
Revenues	1,498
Gain (loss) on derivative outside scope of Topic 606	$ 1,200