Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Conservation Programs [Abstract]
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs (in months)	24 months
Property, Plant and Equipment and Depreciation [Abstract]
Depreciation expense expressed as a percentage of average depreciable property (in hundredths)	2.50%	2.60%	2.70%
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