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Summary of Significant Accounting Policies - Property, Plant and Equipment and Intangible Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Major components of depreciable plant and equipment [Abstract]      
Equipment $ 598,285 $ 567,904  
Transmission and distribution 1,802,008 1,699,953  
Office buildings and other structures 981,615 930,203  
Total depreciable plant and equipment $ 3,381,908 $ 3,198,060  
Useful Lives      
Depreciation expense as percentage of beginning of year balance of depreciable plant 3.20% 3.20% 3.80%
Capital expenditures incurred but not yet paid $ 20,579 $ 24,571  
Capitalized interest costs 4,422 4,133 $ 4,323
Equity funds capitalized 1,926 1,249 198
Water Plant      
Useful Lives      
Depreciation $ 91,906 $ 86,823 $ 63,785
Minimum      
Useful Lives      
Equipment 5 years    
Transmission and distribution plant 35 years    
Office buildings and other structures 7 years    
Useful life of intangible assets 5 years    
Maximum      
Useful Lives      
Equipment 35 years    
Transmission and distribution plant 75 years    
Office buildings and other structures 50 years    
Useful life of intangible assets 70 years