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Benefit Plans - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plans [Abstract]      
Market gains (losses) $ (5,464,000) $ 4,485,000  
Actuarial gain (loss), discount rate change 46,443,000    
Actuarial gain (loss), data and other changes 489,000    
Actuarial gain (loss), mortality change (2,139,000)    
Value of life insurance contracts 4,311,000 3,829,000  
Flexible Spending Plan [Abstract]      
Annual maximum contribution limit under Health Care Spending Account plan 2,500    
Annual maximum contribution limit under Dependent Care Spending Account plan 5,000    
Deferral Plan [Abstract]      
Employer contributions to deferral plan 2,824,000 2,046,000 $ 1,569,000
Executive Supplemental Retirement Plan      
Pension Plans [Abstract]      
Projected benefit obligation 50,757,000 41,768,000  
Net periodic pension cost 4,480,000 3,219,000 $ 2,905,000
Total investments made to fund SERP 7,470,000 7,070,000  
Pension Plan      
Pension Plans [Abstract]      
Net periodic benefit cost, expected return on plan assets     7.00%
Projected benefit obligation 386,091,000 338,240,000 $ 187,877,000
Net periodic pension cost 8,440,000 8,367,000 7,451,000
Total investments made to fund SERP $ 16,609,000 $ 9,476,000  
Minimum      
Pension Plans [Abstract]      
Period for performance standards 3 years    
Minimum | Pension Plan      
Pension Plans [Abstract]      
Net periodic benefit cost, expected return on plan assets 6.50% 7.00%  
Maximum      
Pension Plans [Abstract]      
Period for performance standards 5 years    
Maximum | Pension Plan      
Pension Plans [Abstract]      
Net periodic benefit cost, expected return on plan assets 6.75% 7.25%  
Executives and Non-employee Directors | Deferred compensation arrangement with individual, by type of compensation, pension and other postretirement benefits      
Special Deferral Election Plan and Deferral Election Program [Abstract]      
Deferrals by executives and non-employee directors $ 6,144,000 $ 7,834,000 $ 4,244,000
Executives and Non-employee Directors | Deferred compensation arrangement with individual, by type of compensation, pension and other postretirement benefits | Connecticut Water Service, Inc.      
Special Deferral Election Plan and Deferral Election Program [Abstract]      
Deferrals by executives and non-employee directors $ 4,130,000 $ 3,801,000  
CTWS Employees      
Pension Plans [Abstract]      
Rate of compensation increase to company sponsored savings plan 1.50%