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Summary of Significant Accounting Policies (Utility Plant) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]      
Capitalized interest costs $ 1,112us-gaap_InterestCostsCapitalized $ 905us-gaap_InterestCostsCapitalized $ 913us-gaap_InterestCostsCapitalized
Construction in progress 23,208us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress 30,846us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress  
Land 16,838us-gaap_PublicUtilitiesPropertyPlantAndEquipmentLand 10,418us-gaap_PublicUtilitiesPropertyPlantAndEquipmentLand  
Major components of depreciable plant and equipment [Abstract]      
Equipment 242,383us-gaap_PublicUtilitiesPropertyPlantAndEquipmentEquipment 226,557us-gaap_PublicUtilitiesPropertyPlantAndEquipmentEquipment  
Transmission and distribution 1,042,794us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution 967,031us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution  
Office buildings and other structures 68,595us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment 60,998us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment  
Total depreciable plant and equipment 1,353,772us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService 1,254,586us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService  
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation expense as percentage of beginning of year balance of depreciable plant 3.40%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService 3.40%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService 3.50%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService
Minimum [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Estimated service lives of assets 5 years    
Estimated useful life of equipment 5 years    
Estimated useful life of transmission and distribution plant 35 years    
Estimated useful lives of office buildings and other structures 7 years    
Useful life of intangible assets 5 years    
Maximum [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Estimated service lives of assets 75 years    
Estimated useful life of equipment 35 years    
Estimated useful life of transmission and distribution plant 75 years    
Estimated useful lives of office buildings and other structures 50 years    
Useful life of intangible assets 70 years    
General and Administrative Expense [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Depreciation 1,586us-gaap_Depreciation
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember		 1,556us-gaap_Depreciation
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember		 1,531us-gaap_Depreciation
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
Water Plant [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Depreciation $ 35,424us-gaap_Depreciation
/ us-gaap_PublicUtilityAxis
= us-gaap_WaterPlantMember		 $ 32,616us-gaap_Depreciation
/ us-gaap_PublicUtilityAxis
= us-gaap_WaterPlantMember		 $ 31,005us-gaap_Depreciation
/ us-gaap_PublicUtilityAxis
= us-gaap_WaterPlantMember