Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 11,537	$ 11,404	$ 11,107
Investment in subsidiaries	$ 141				$ 140				141	140
Depreciation and amortization									1,642	1,479	1,303
Total interest charges and financing costs									652	628	620
Income tax expense (benefit)									181	542	581
Net income (loss)	$ 214	$ 491	$ 265	$ 291	$ 189	$ 492	$ 227	$ 239	1,261	1,148	1,123
Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									9,720	9,678	9,501
Depreciation and amortization									1,421	1,298	1,136
Total interest charges and financing costs									449	449	450
Income tax expense (benefit)									187	528	567
Net income (loss)									1,177	1,066	1,067
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,741	1,651	1,532
Depreciation and amortization									212	174	160
Total interest charges and financing costs									61	57	54
Income tax expense (benefit)									28	23	76
Net income (loss)									187	182	124
All Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									79	78	76
Depreciation and amortization									9	7	7
Total interest charges and financing costs									142	122	116
Income tax expense (benefit)									(34)	(9)	(62)
Net income (loss)									(103)	(100)	(68)
Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									11,540	11,407	11,109
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									9,719	9,676	9,500
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,739	1,650	1,531
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									(3)	(3)	(2)
Intersegment Eliminations [Member] | Regulated Electric
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1	2	1
Intersegment Eliminations [Member] | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 2	$ 1	$ 1