Regulation and Rates Power Cost Adjustment Mechanism (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Customers’ Share | Plus/Minus $20 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	0.00%
Customers’ Share | Plus/Minus $20 million - $40 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	50.00%
Customers’ Share | Plus/Minus $40 million - $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	90.00%
Customers’ Share | Plus/Minus $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	95.00%
Company's Share | Plus/Minus $20 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	100.00%
Company's Share | Plus/Minus $20 million - $40 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	50.00%
Company's Share | Plus/Minus $40 million - $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	10.00%
Company's Share | Plus/Minus $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability	5.00%
Minimum
Regulatory Assets [Line Items]
Annual Power Cost Variability	$ 20.0
Minimum | Plus/Minus $20 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	20
Minimum | Plus/Minus $20 million - $40 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	20
Minimum | Plus/Minus $40 million - $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	40
Maximum
Regulatory Assets [Line Items]
Annual Power Cost Variability	40.0
Maximum | Plus/Minus $20 million - $40 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	40
Maximum | Plus/Minus $40 million - $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	120
Maximum | Plus/Minus $120 million
Regulatory Assets [Line Items]
Annual Power Cost Variability, Graduated Scale, Range	$ 120