Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue Recognition [Abstract]
Number of years between rate review processes (in years)	2 years
Minimum annual tolerance band percentage for future rate recovery or refund of fuel costs (in hundredths)	2.00%
Property, Plant and Equipment and Depreciation [Abstract]
Depreciation expense expressed as a percentage of average depreciable property (in hundredths)	3.50%	3.60%	3.50%
Cash and Cash Equivalents [Abstract]
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents (in months)	3 months