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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% |  |

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