Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 3,750	$ 3,550	$ 3,159	$ 3,753	$ 3,271	$ 3,245	$ 2,855	$ 3,236	$ 14,212	$ 12,607	$ 10,630
Electric
Disaggregation of Revenue [Line Items]
Revenues									5,298
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues									20
Electric | Alternative Revenue Programs
Disaggregation of Revenue [Line Items]
Revenues									21
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues									2,494
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues									1,794
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues									690
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues									320
Gas
Disaggregation of Revenue [Line Items]
Revenues									1,436
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues									7
Gas | Alternative Revenue Programs
Disaggregation of Revenue [Line Items]
Revenues									2
Gas | Other
Disaggregation of Revenue [Line Items]
Revenues									91
Gas | Gas sales
Disaggregation of Revenue [Line Items]
Revenues									1,055
Gas | End User Transportation
Disaggregation of Revenue [Line Items]
Revenues									232
Gas | Intermediate Transportation
Disaggregation of Revenue [Line Items]
Revenues									58
Gas Storage and Pipelines
Disaggregation of Revenue [Line Items]
Revenues									485
Power and Industrial Projects
Disaggregation of Revenue [Line Items]
Revenues									2,204
Lease revenue outside scope of Topic 606									125
Energy Trading
Disaggregation of Revenue [Line Items]
Revenues									5,557
Gain (loss) on derivative outside scope of Topic 606									$ 4,500