Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Regulated electric		$ 228	$ 233	$ 625	$ 589
Regulated natural gas		18	18	94	83
Revenues		246	251	719	672
Cost of fuel, energy and capacity		96	103	294	269
Natural gas purchased for resale		7	7	57	48
Cost of Sales		103	110	351	317
Gross Margin		143	141	368	355
Operating and maintenance		41	47	116	120
Depreciation and amortization		28	27	84	79
Operating Income (Loss)		66	60	146	137
Interest Expense		16	16	46	46
Assets		3,493		3,493		$ 3,388
Electric Operations [Member]
Segment Reporting Information [Line Items]
Gross Margin		132	130	331	320
Operating and maintenance		36	42	103	106
Depreciation and amortization		24	23	72	68
Operating Income (Loss)		65	59	136	129
Interest Expense		14	14	42	42
Assets		3,017		3,017		3,031
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Gross Margin		11	11	37	35
Operating and maintenance		5	5	13	14
Depreciation and amortization		4	4	12	11
Operating Income (Loss)		1	1	10	8
Interest Expense		2	$ 2	4	$ 4
Assets		316		316		327
Regulated common assets [Member]
Segment Reporting Information [Line Items]
Assets	[1]	$ 160		$ 160		$ 30

[1] (1)Consists principally of cash and cash equivalents not included in either the regulated electric or regulated natural gas segments.