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Summary of Significant Accounting Policies (Utility Plant) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Public Utility, Property, Plant and Equipment [Line Items]      
Capitalized interest costs $ 1,188 $ 1,112 $ 905
Construction in progress 45,573 23,208  
Major components of depreciable plant and equipment [Abstract]      
Equipment 254,940 242,383  
Transmission and distribution 1,108,659 1,042,794  
Office buildings and other structures 74,722 68,595  
Total depreciable plant and equipment $ 1,438,321 $ 1,353,772  
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation expense as percentage of beginning of year balance of depreciable plant 3.40% 3.40% 3.40%
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    
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    
General and Administrative Expense [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation $ 1,590 $ 1,586 $ 1,556
Water Plant [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation $ 38,233 $ 35,424 $ 32,616