Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 3,514	$ 3,753
Electric
Disaggregation of Revenue [Line Items]
Revenues	1,235	1,205
Reduction of revenue due to Tax Cuts and Jobs Act		39
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	4	5
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues	553	586
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues	421	429
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues	163	176
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	98	14
Gas
Disaggregation of Revenue [Line Items]
Revenues	645	550
Reduction of revenue due to Tax Cuts and Jobs Act		32
Gas | Alternative Revenue Programs
Disaggregation of Revenue [Line Items]
Revenues		3
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	2	2
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	61	(10)
Gas | Gas sales
Disaggregation of Revenue [Line Items]
Revenues	477	457
Gas | End User Transportation
Disaggregation of Revenue [Line Items]
Revenues	81	85
Gas | Intermediate Transportation
Disaggregation of Revenue [Line Items]
Revenues	26	18
Gas Storage and Pipelines
Disaggregation of Revenue [Line Items]
Revenues	116	119
Lease revenue outside scope of Topic 606	2
Power and Industrial Projects
Disaggregation of Revenue [Line Items]
Revenues	388	567
Lease revenue outside scope of Topic 606	31	28
Energy Trading
Disaggregation of Revenue [Line Items]
Revenues	1,301	1,498
Gain (loss) on derivative outside scope of Topic 606	$ 926	$ 1,200