Summary of Significant Accounting Policies (Utility Plant) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Public Utility, Property, Plant and Equipment [Line Items]
Capitalized interest costs	$ 913	$ 699	$ 555
Construction in progress	24,298	18,527
Major components of depreciable plant and equipment [Abstract]
Equipment	214,670	202,181
Transmission and distribution	892,957	811,332
Office buildings and other structures	58,593	56,503
Total depreciable plant and equipment	1,166,220	1,070,016
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation expense as percentage of beginning of year balance of depreciable plant	3.50%	3.50%	3.50%
Minimum [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Estimated service lives of assets	P5Y
Estimated useful life of equipment	P5Y
Estimated useful life of transmission and distribution plant	P35Y
Estimated useful lives of office buildings and other structures	P7Y
Useful life of intangible assets	5 years
Maximum [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Estimated service lives of assets	P75Y
Estimated useful life of equipment	P35Y
Estimated useful life of transmission and distribution plant	P75Y
Estimated useful lives of office buildings and other structures	P50Y
Useful life of intangible assets	70 years
General and Administrative Expense [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation	1,531	1,516	1,425
Water Plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation	$ 31,005	$ 29,141	$ 26,331