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]
Operating revenue, Regulated Natural Gas										$ 1,024	$ 1,161	$ 1,015
Unregulated Operating Revenue										43	43	40
Regulated and Unregulated Operating Revenue		$ 1,134	$ 1,081	$ 911	$ 1,057	$ 1,060	$ 1,044	$ 910	$ 1,223	4,183	4,237	4,086
Depreciation and amortization										655	602	561
Interest charges and financing costs										224	224	186
Income tax expense (benefit)										45	80	113
Net income (loss)		$ 133	$ 218	$ 108	$ 129	$ 133	$ 204	$ 102	$ 139	588	578	552
Intercompany Revenue										5	5	4
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										3,117	3,034	3,031
Depreciation and amortization										497	455	416
Interest charges and financing costs										173	173	142
Income tax expense (benefit)										13	45	103
Net income (loss)										460	465	429
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues										1,024	1,161	1,016
Depreciation and amortization										152	141	140
Interest charges and financing costs										50	50	43
Income tax expense (benefit)										29	33	13
Net income (loss)										126	119	121
All Other
Segment Reporting Information [Line Items]
Depreciation and amortization										6	6	5
Interest charges and financing costs										1	1	1
Income tax expense (benefit)										3	2	(3)
Net income (loss)										2	(6)	2
Operating Segments
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]									4,184	4,238	4,087
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Electric										3,116	3,033	3,031
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenue, Regulated Natural Gas										1,024	1,161	1,015
Operating Segments | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	[1]									43	43	40
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue										(1)	(1)	(1)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Electric										1	1	0
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenue, Regulated Natural Gas										$ 0	$ 0	$ 1

[1] Operating revenues include $5 million, $5 million and $4 million of intercompany revenue for the years ended Dec. 31, 2020, 2019 and 2018, respectively. See Note 13 for further information.