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Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
Segment Information (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 141		$ 140
Operating revenues	2,951	$ 2,946
Net income (loss)	291	239
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,270	2,299
Net income (loss)	219	194
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	141		$ 140
Operating revenues	662	626
Net income (loss)	95	63
All Other
Segment Reporting Information [Line Items]
Operating revenues	19	21
Net income (loss)	(22)	(18)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	2,951	2,946
Operating Segments \| Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	2,270	2,299
Operating Segments \| Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	662	626
Operating Segments \| All Other
Segment Reporting Information [Line Items]
Operating revenues	19	21
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	0	0
Net income (loss)	(1)	0
Intersegment Eliminations \| Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	0	0
Intersegment Eliminations \| Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	0	0
Intersegment Eliminations \| All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0