Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Investment in subsidiaries	$ 155				$ 141				$ 155	$ 141
Operating revenues from external customers, Regulated Electric									9,575	9,719	$ 9,676
Operating revenues from external customers, Regulated Natural Gas									1,868	1,739	1,650
Unregulated Operating Revenue									86	79	78
Regulated and Unregulated Operating Revenue	2,798	$ 3,013	$ 2,577	$ 3,141	2,880	$ 3,048	$ 2,658	$ 2,951	11,529	11,537	11,404
Depreciation and amortization									1,765	1,642	1,479
Interest charges and financing costs									736	652	628
Income Tax Expense (Benefit)									128	181	542
Net income (loss)	$ 292	$ 527	$ 238	$ 315	$ 214	$ 491	$ 265	$ 291	1,372	1,261	1,148
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues									9,576	9,720	9,678
Depreciation and amortization									1,535	1,421	1,298
Interest charges and financing costs									500	449	449
Income Tax Expense (Benefit)									125	187	528
Net income (loss)									1,288	1,177	1,066
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues									1,870	1,741	1,651
Depreciation and amortization									219	212	174
Interest charges and financing costs									69	61	57
Income Tax Expense (Benefit)									48	28	23
Net income (loss)									195	187	182
All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue									86	79	78
Depreciation and amortization									11	9	7
Interest charges and financing costs									167	142	122
Income Tax Expense (Benefit)									(45)	(34)	(9)
Net income (loss)									(111)	(103)	(100)
Operating Segments
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue									11,532	11,540	11,407
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues from external customers, Regulated Electric									9,575	9,719	9,676
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues from external customers, Regulated Natural Gas									1,868	1,739	1,650
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue									(3)	(3)	(3)
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues from external customers, Regulated Electric									1	1	2
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues from external customers, Regulated Natural Gas									$ 2	$ 2	$ 1