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Summary of Significant Accounting Policies (Utility Plant) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]      
Capitalized interest costs $ 2,188 $ 1,188 $ 1,112
Construction in progress 70,453 45,573  
Major components of depreciable plant and equipment [Abstract]      
Equipment 269,734 254,940  
Transmission and distribution 1,204,520 1,108,659  
Office buildings and other structures 79,762 74,722  
Total depreciable plant and equipment $ 1,554,016 $ 1,438,321  
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation expense as percentage of beginning of year balance of depreciable plant 3.50% 3.40% 3.40%
Minimum [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
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    
Estimated service lives of assets 5 years    
Maximum [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
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    
Estimated service lives of assets 75 years    
General and Administrative Expense [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation $ 1,670 $ 1,590 $ 1,586
Water Plant [Member]      
Estimated Useful Lives of Plant and Equipment [Abstract]      
Depreciation $ 42,659 $ 38,722 $ 35,918