Segments and Related Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Unregulated Operating Revenue										$ 0	$ 0	$ 1
Regulated and Unregulated Operating Revenue		$ 248	$ 251	$ 218	$ 257	$ 232	$ 244	$ 229	$ 276	974	981	1,022
Depreciation and amortization										156	138	126
Interest charges and financing costs										38	38	35
Income Tax Expense (Benefit)										(7)	16	32
Net income		$ 21	$ 40	$ 12	$ 34	$ 17	$ 29	$ 9	$ 24	107	79	98
Regulated Operating Revenue, Electric, Affiliate										168	177	158
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										856	845	879
Depreciation and amortization										108	109	98
Interest charges and financing costs										34	34	32
Income Tax Expense (Benefit)										12	13	28
Net income										98	68	85
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										119	136	142
Depreciation and amortization										48	29	28
Interest charges and financing costs										4	4	3
Income Tax Expense (Benefit)										(20)	2	5
Net income										7	10	13
All Other
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)										1	1	(1)
Net income										2	1	0
Operating Segments
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]									975	981	1,022
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenue, Regulated Electric	[1]									855	845	879
Operating Segments | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	[1]									0	0	1
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue										(1)	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenue, Regulated Electric										$ 1	$ 0	$ 0

[1] (a)    Operating revenues include $168 million, $177 million and $158 million of intercompany revenue for the years ended Dec. 31, 2020, 2019 and 2018, respectively. See Note 12 for further information.