Revenues (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 281	$ 257
Regulated Electric \| Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	224	212
Regulated Natural Gas \| Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	57	45
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	230	211
Retail \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	102	90
Retail \| C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	126	119
Retail \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	2
Retail \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	175	167
Retail \| Regulated Electric \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	71	65
Retail \| Regulated Electric \| C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	102	100
Retail \| Regulated Electric \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	2
Retail \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	55	44
Retail \| Regulated Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	31	25
Retail \| Regulated Natural Gas \| C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	24	19
Retail \| Regulated Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Interchange
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46	41
Interchange \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46	41
Interchange \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	1
Other \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	1
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	277	253
Total revenue from contracts with customers \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	221	208
Total revenue from contracts with customers \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	56	45
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	4	4
Alternative revenue and other \| Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3	4
Alternative revenue and other \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 1	$ 0

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