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Pension Plans and Other Postretirement Benefit Plans - Additional Information (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Payment for pension benefits | $ 22,000 | $ 22,000 | $ 22,000 |
Percentage of service related net periodic benefit costs capitalized to utility property | 40.00% |  |  |
Percentage of service related net periodic benefit costs recorded to operating expenses | 60.00% |  |  |
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] | Executive Officer [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Deferred compensation, earlier of retirement, termination, disability or death, percent | 75.00% |  |  |
Deferred compensation incentive payments, percent | 100.00% |  |  |
Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 35.00% | 35.00% |  |
Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 49.00% | 49.00% |  |
Real Estate [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 7.00% | 7.00% |  |
Absolute Return [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 9.00% | 9.00% |  |
Pension Plans, Defined Benefit [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected contributions to pension plan | $ 42,000 |  |  |
Other Postretirement Benefits [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected contributions to pension plan | $ 6,800 |  |  |
Other Postretirement Benefits [Member] | Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 60.00% | 60.00% |  |
Other Postretirement Benefits [Member] | Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation | 40.00% | 40.00% |  |
Other Postretirement Benefits [Member] | Forecast [Member] | Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation |  |  |  | 55.00%
Other Postretirement Benefits [Member] | Forecast [Member] | Debt Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation |  |  |  | 40.00%
Other Postretirement Benefits [Member] | Forecast [Member] | Real Estate [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation |  |  |  | 5.00%
Other Postretirement Benefits [Member] | Forecast [Member] | Absolute Return [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target investment allocation |  |  |  | 0.00%

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