Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 255	$ 218	$ 536	$ 475
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	224	199	448	411
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	31	19	88	64
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	204	172	434	383
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	79	70	181	160
Retail | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	123	100	250	220
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	2	3	3
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	174	155	349	322
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	64	60	135	125
Retail | Regulated Electric | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	108	93	211	194
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	2	3	3
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	30	17	85	61
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15	10	46	35
Retail | Regulated Natural Gas | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15	7	39	26
Retail | Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Interchange
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46	41	92	82
Interchange | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46	41	92	82
Interchange | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2	1	3	3
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	0	1	1
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	1	2	2
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	252	214	529	468
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	221	196	442	405
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	31	18	87	63
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3	4	7	7
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3	3	6	6
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0	$ 1	$ 1	$ 1