Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014 TheNumberOfReportableSegments		Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Number of reportable segments				2
Electric	$ 233		$ 213	$ 589		$ 559
Natural gas	18		12	83		61
Revenues	251		225	672		620
Cost of fuel, energy and capacity	103		80	269		212
Natural gas purchased for resale	7		4	48		28
Gross margin	141		141	355		380
Operating and maintenance expense	47		52	120		148
Depreciation and amortization	27		26	79		78
Property and other taxes	7		6	19		19
Merger-related expenses	0		2	0		6
Interest expense, net of allowance for debt funds	15		15	44		47
Allowance for equity funds	(1)		(1)	(3)		(2)
Other, net	(3)		(1)	(8)		(8)
Income before income tax expense	49		42	104		92
Assets	3,443			3,443			3,369
Electric [Member]
Segment Reporting Information [Line Items]
Gross margin	130		133	320		347
Assets	2,976			2,976			2,957
Natural Gas [Member]
Segment Reporting Information [Line Items]
Gross margin	11		8	35		33
Assets	329			329			335
Other [Member]
Segment Reporting Information [Line Items]
Assets	$ 138	[1]		$ 138	[1]		$ 77	[1]

[1] Consists principally of cash and cash equivalents.