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Pay vs Performance Disclosure | 12 Months Ended
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Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Pay vs Performance Disclosure |  |  |  |
Pay vs Performance Disclosure, Table | ​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Value of Initial Fixed $100 Investment Based on | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | Fiscal Year | ​ | ​ | SCT Total PEO #1(1) | ​ | ​ | CAP PEO #1(1) | ​ | ​ | SCT Total PEO #2(1) | ​ | ​ | CAP PEO #2(1) | ​ | ​ | Average SCT Non-PEO NEOs(2) | ​ | ​ | CAP Non-PEO NEOs(2) | ​ | ​ | Company TSR(3) | ​ | ​ | Peer Group TSR(3) | ​ | ​ | Net Income | ​ | ​ | Company Selected Measure (EPS Non-GAAP)(4) | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​ | ​ | j | ​ | ​ | k | ​
​ | 1 (2023) | ​ | ​ | ​ | ​ | 18,885,577 | ​ | ​ | ​ | ​ | ​ | 7,429,247 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 2,936,041 | ​ | ​ | ​ | ​ | ​ | 1,135,182 | ​ | ​ | ​ | ​ | ​ | 82 | ​ | ​ | ​ | ​ | ​ | 107 | ​ | ​ | ​ | ​ | ​ | -442.2 | ​ | ​ | ​ | ​ | ​ | 4.34 | ​ | ​
​ | 2 (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​ | ​ | ​ | ​ | 107 | ​ | ​ | ​ | ​ | ​ | 117 | ​ | ​ | ​ | ​ | ​ | 1,404.9 | ​ | ​ | ​ | ​ | ​ | 4.09 | ​ | ​
​ | 3 (2021) | ​ | ​ | ​ | ​ | 6,467,078 | ​ | ​ | ​ | ​ | ​ | 5,120,192 | ​ | ​ | ​ | ​ | ​ | 10,220,941 | ​ | ​ | ​ | ​ | ​ | 10,910,465 | ​ | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​ | ​ | ​ | ​ | 113 | ​ | ​ | ​ | ​ | ​ | 116 | ​ | ​ | ​ | ​ | ​ | 1,220.5 | ​ | ​ | ​ | ​ | ​ | 3.86 | ​ | ​
​ | 4 (2020) | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 14,575,276 | ​ | ​ | ​ | ​ | ​ | 14,062,876 | ​ | ​ | ​ | ​ | ​ | 5,002,344 | ​ | ​ | ​ | ​ | ​ | 3,960,432 | ​ | ​ | ​ | ​ | ​ | 105 | ​ | ​ | ​ | ​ | ​ | 99 | ​ | ​ | ​ | ​ | ​ | 1,205.2 | ​ | ​ | ​ | ​ | ​ | 3.64 | ​ | ​
​ (1) The PEOs for each year are as follows: ​ a. 2023, 2022 and 2021 — Joseph R. Nolan, Jr. ​ b. 2020 and 2021 — James J. Judge. Mr. Nolan became the Company’s Chief Executive Officer on May 5, 2021 ​ (2) The Non-PEO NEOs for each year are as follows: ​ a. 2023 — John M. Moreira, Gregory B. Butler, Christine M. Carmody, James W. Hunt, III and Werner J. Schweiger ​ b. 2022 — John M. Moreira, Gregory B. Butler, Christine M. Carmody, Werner J. Schweiger, James J. Judge and Philip J. Lembo ​ c. 2021 — Philip J. Lembo, Werner J. Schweiger, Gregory B. Butler and Christine M. Carmody ​ d. 2020 — Philip J. Lembo, Werner J. Schweiger, Gregory B. Butler and Joseph R. Nolan, Jr. ​ (3) The values disclosed in this TSR column represent the measurement period value of an investment of $100 in our units as of December 31, 2020, and then valued again on each of December 31, 2021, December 31, 2022 and December 31, 2023 as compared to the EEI Index, as disclosed in our Annual Report on Form 10-K for fiscal year ended 2023 pursuant to Item 201(e) of Regulation S-K ​ (4) Set forth in Footnote 1 on page 33 and Exhibit A to the CD&A is a full description of how non-GAAP EPS was determined. The Company believes non-GAAP EPS is a very important measure to investors and which is indicative of Company performance; accordingly, it represents a significant factor in determining both annual and long term executive compensation, though as noted the Compensation Committee determined that due to the disappointing performance in our 2023 stock price, the Committee scored the EPS goal at zero out of 200 within the EPS based portion of the 2023 annual incentive program ​ To calculate CAP for our PEOs and Non-PEO NEOs in the table above, the following amounts were deducted and added to the values as reported in the SCT:
​ | Fiscal Year | ​ | ​ | SCT Total PEO #1 | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | CAP PEO #1 | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | 1 (2023) | ​ | ​ | ​ | ​ | 18,885,577 | ​ | ​ | ​ | ​ | ​ | -7,832,472 | ​ | ​ | ​ | ​ | ​ | 689,828 | ​ | ​ | ​ | ​ | ​ | -8,018,396 | ​ | ​ | ​ | ​ | ​ | 6,023,948 | ​ | ​ | ​ | ​ | ​ | -182,331 | ​ | ​ | ​ | ​ | ​ | -2,136,906 | ​ | ​ | ​ | ​ | ​ | 7,429,247 | ​ | ​
​ | 2 (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | -2,143,443 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | -6,825,923 | ​ | ​ | ​ | ​ | ​ | 6,749,195 | ​ | ​ | ​ | ​ | ​ | -235,802 | ​ | ​ | ​ | ​ | ​ | -74,557 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​
​ | 3 (2021) | ​ | ​ | ​ | ​ | 6,467,078 | ​ | ​ | ​ | ​ | ​ | -1,705,782 | ​ | ​ | ​ | ​ | ​ | 336,850 | ​ | ​ | ​ | ​ | ​ | -1,441,650 | ​ | ​ | ​ | ​ | ​ | 1,546,319 | ​ | ​ | ​ | ​ | ​ | -201,335 | ​ | ​ | ​ | ​ | ​ | 118,712 | ​ | ​ | ​ | ​ | ​ | 5,120,192 | ​ | ​
​ | Fiscal Year | ​ | ​ | SCT Total PEO #2 | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | CAP PEO #2 | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | (2021) | ​ | ​ | ​ | ​ | 10,220,941 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 845,242 | ​ | ​ | ​ | ​ | ​ | -6,786,337 | ​ | ​ | ​ | ​ | ​ | 7,279,051 | ​ | ​ | ​ | ​ | ​ | -1,266,246 | ​ | ​ | ​ | ​ | ​ | 617,814 | ​ | ​ | ​ | ​ | ​ | 10,910,465 | ​ | ​
​ | (2020) | ​ | ​ | ​ | ​ | 14,575,276 | ​ | ​ | ​ | ​ | ​ | -3,742,215 | ​ | ​ | ​ | ​ | ​ | 111,326 | ​ | ​ | ​ | ​ | ​ | -6,682,612 | ​ | ​ | ​ | ​ | ​ | 6,707,152 | ​ | ​ | ​ | ​ | ​ | 2,047,648 | ​ | ​ | ​ | ​ | ​ | 1,046,301 | ​ | ​ | ​ | ​ | ​ | 14,062,876 | ​ | ​
​ | Fiscal Year | ​ | ​ | Average SCT Non-PEO NEOs | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | Average CAP Non-PEO NEOs | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | 1 (2023) | ​ | ​ | ​ | ​ | 2,936,041 | ​ | ​ | ​ | ​ | ​ | -478,864 | ​ | ​ | ​ | ​ | ​ | 84,416 | ​ | ​ | ​ | ​ | ​ | -1,451,776 | ​ | ​ | ​ | ​ | ​ | 827,273 | ​ | ​ | ​ | ​ | ​ | -102,269 | ​ | ​ | ​ | ​ | ​ | -679,639 | ​ | ​ | ​ | ​ | ​ | 1,135,182 | ​ | ​
​ | 2 (2022) | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | -69,941 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | -2,054,232 | ​ | ​ | ​ | ​ | ​ | 2,031,140 | ​ | ​ | ​ | ​ | ​ | -419,104 | ​ | ​ | ​ | ​ | ​ | -109,907 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​
​ | 3 (2021) | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | -1,205,814 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | -1,408,117 | ​ | ​ | ​ | ​ | ​ | 1,510,350 | ​ | ​ | ​ | ​ | ​ | -238,540 | ​ | ​ | ​ | ​ | ​ | 125,057 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​
​ | 4 (2020) | ​ | ​ | ​ | ​ | 5,002,344 | ​ | ​ | ​ | ​ | ​ | -1,929,875 | ​ | ​ | ​ | ​ | ​ | 257,161 | ​ | ​ | ​ | ​ | ​ | -1,494,123 | ​ | ​ | ​ | ​ | ​ | 1,499,609 | ​ | ​ | ​ | ​ | ​ | 422,302 | ​ | ​ | ​ | ​ | ​ | 203,013 | ​ | ​ | ​ | ​ | ​ | 3,960,432 | ​ | ​
​ (a) The total pension benefit adjustments for each applicable year include the aggregate of two components: (i) the actuarially determined service cost for services rendered by the PEO and non-PEO NEOs during the applicable year (the “service cost”); and (ii) the entire cost of benefits granted in a plan amendment (or initiation) during the applicable year that are attributed by the benefit formula to services rendered in periods prior to the plan amendment or initiation (the “prior service cost”), in each case, calculated in accordance with U.S. GAAP ​
​ | Fiscal Year | ​ | ​ | Service Cost | ​ | ​ | Prior Service Cost | ​ | ​ | Total Pension Benefit Adjustments | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​
​ | PEO#1 (2023) | ​ | ​ | ​ | ​ | 689,828 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 689,828 | ​ | ​
​ | PEO#1 (2022) | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​
​ | PEO#1 (2021) | ​ | ​ | ​ | ​ | 336,850 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 336,850 | ​ | ​
​ | PEO#2 (2021) | ​ | ​ | ​ | ​ | 845,242 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 845,242 | ​ | ​
​ | PEO#2 (2020) | ​ | ​ | ​ | ​ | 111,326 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 111,326 | ​ | ​
​ | Average Non-PEO NEO (2023) | ​ | ​ | ​ | ​ | 84,416 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 84,416 | ​ | ​
​ | Average Non-PEO NEO (2022) | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​
​ | Average Non-PEO NEO (2021) | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​
​ | Average Non-PEO NEO (2020) | ​ | ​ | ​ | ​ | 257,161 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 257,161 | ​ | ​
​ (b) For the periods covered, no individual equity awards were granted and vested in the same year, except as it relates to the 2023 awards to Mr. Schweiger, who retired on July 1, 2023, respectively, and no dividends or other earnings were paid on outstanding and unvested awards. The valuation assumptions used to calculate the fair values of performance shares reflect the probable outcome of the performance conditions as of the applicable measuring date (or actual performance results approved by the Compensation Committee as of the applicable vesting date). Outstanding RSUs accrue dividend equivalent units in the form of additional common shares that are not distributed unless and until the underlying award vests and becomes payable. The value of dividend equivalent units are reflected in the values set forth above. |  |  |
Company Selected Measure Name | EPS |  |  |
Named Executive Officers, Footnote | (1) The PEOs for each year are as follows: ​ a. 2023, 2022 and 2021 — Joseph R. Nolan, Jr. ​ b. 2020 and 2021 — James J. Judge. Mr. Nolan became the Company’s Chief Executive Officer on May 5, 2021 ​ (2) The Non-PEO NEOs for each year are as follows: ​ a. 2023 — John M. Moreira, Gregory B. Butler, Christine M. Carmody, James W. Hunt, III and Werner J. Schweiger ​ b. 2022 — John M. Moreira, Gregory B. Butler, Christine M. Carmody, Werner J. Schweiger, James J. Judge and Philip J. Lembo ​ c. 2021 — Philip J. Lembo, Werner J. Schweiger, Gregory B. Butler and Christine M. Carmody ​ d. 2020 — Philip J. Lembo, Werner J. Schweiger, Gregory B. Butler and Joseph R. Nolan |  |  |
Peer Group Issuers, Footnote | (3) The values disclosed in this TSR column represent the measurement period value of an investment of $100 in our units as of December 31, 2020, and then valued again on each of December 31, 2021, December 31, 2022 and December 31, 2023 as compared to the EEI Index, as disclosed in our Annual Report on Form 10-K for fiscal year ended 2023 pursuant to Item 201(e) of Regulation S-K |  |  |
Adjustment To PEO Compensation, Footnote | To calculate CAP for our PEOs and Non-PEO NEOs in the table above, the following amounts were deducted and added to the values as reported in the SCT:
​ | Fiscal Year | ​ | ​ | SCT Total PEO #1 | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | CAP PEO #1 | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | 1 (2023) | ​ | ​ | ​ | ​ | 18,885,577 | ​ | ​ | ​ | ​ | ​ | -7,832,472 | ​ | ​ | ​ | ​ | ​ | 689,828 | ​ | ​ | ​ | ​ | ​ | -8,018,396 | ​ | ​ | ​ | ​ | ​ | 6,023,948 | ​ | ​ | ​ | ​ | ​ | -182,331 | ​ | ​ | ​ | ​ | ​ | -2,136,906 | ​ | ​ | ​ | ​ | ​ | 7,429,247 | ​ | ​
​ | 2 (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | -2,143,443 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | -6,825,923 | ​ | ​ | ​ | ​ | ​ | 6,749,195 | ​ | ​ | ​ | ​ | ​ | -235,802 | ​ | ​ | ​ | ​ | ​ | -74,557 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​
​ | 3 (2021) | ​ | ​ | ​ | ​ | 6,467,078 | ​ | ​ | ​ | ​ | ​ | -1,705,782 | ​ | ​ | ​ | ​ | ​ | 336,850 | ​ | ​ | ​ | ​ | ​ | -1,441,650 | ​ | ​ | ​ | ​ | ​ | 1,546,319 | ​ | ​ | ​ | ​ | ​ | -201,335 | ​ | ​ | ​ | ​ | ​ | 118,712 | ​ | ​ | ​ | ​ | ​ | 5,120,192 | ​ | ​
​ | Fiscal Year | ​ | ​ | SCT Total PEO #2 | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | CAP PEO #2 | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | (2021) | ​ | ​ | ​ | ​ | 10,220,941 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 845,242 | ​ | ​ | ​ | ​ | ​ | -6,786,337 | ​ | ​ | ​ | ​ | ​ | 7,279,051 | ​ | ​ | ​ | ​ | ​ | -1,266,246 | ​ | ​ | ​ | ​ | ​ | 617,814 | ​ | ​ | ​ | ​ | ​ | 10,910,465 | ​ | ​
​ | (2020) | ​ | ​ | ​ | ​ | 14,575,276 | ​ | ​ | ​ | ​ | ​ | -3,742,215 | ​ | ​ | ​ | ​ | ​ | 111,326 | ​ | ​ | ​ | ​ | ​ | -6,682,612 | ​ | ​ | ​ | ​ | ​ | 6,707,152 | ​ | ​ | ​ | ​ | ​ | 2,047,648 | ​ | ​ | ​ | ​ | ​ | 1,046,301 | ​ | ​ | ​ | ​ | ​ | 14,062,876 | ​ | ​
(a) The total pension benefit adjustments for each applicable year include the aggregate of two components: (i) the actuarially determined service cost for services rendered by the PEO and non-PEO NEOs during the applicable year (the “service cost”); and (ii) the entire cost of benefits granted in a plan amendment (or initiation) during the applicable year that are attributed by the benefit formula to services rendered in periods prior to the plan amendment or initiation (the “prior service cost”), in each case, calculated in accordance with U.S. GAAP ​
​ | Fiscal Year | ​ | ​ | Service Cost | ​ | ​ | Prior Service Cost | ​ | ​ | Total Pension Benefit Adjustments | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​
​ | PEO#1 (2023) | ​ | ​ | ​ | ​ | 689,828 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 689,828 | ​ | ​
​ | PEO#1 (2022) | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​
​ | PEO#1 (2021) | ​ | ​ | ​ | ​ | 336,850 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 336,850 | ​ | ​
​ | PEO#2 (2021) | ​ | ​ | ​ | ​ | 845,242 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 845,242 | ​ | ​
​ | PEO#2 (2020) | ​ | ​ | ​ | ​ | 111,326 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 111,326 | ​ | ​
​ | Average Non-PEO NEO (2023) | ​ | ​ | ​ | ​ | 84,416 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 84,416 | ​ | ​
​ | Average Non-PEO NEO (2022) | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​
​ | Average Non-PEO NEO (2021) | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​
​ | Average Non-PEO NEO (2020) | ​ | ​ | ​ | ​ | 257,161 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 257,161 | ​ | ​
​ (b) For the periods covered, no individual equity awards were granted and vested in the same year, except as it relates to the 2023 awards to Mr. Schweiger, who retired on July 1, 2023, respectively, and no dividends or other earnings were paid on outstanding and unvested awards. The valuation assumptions used to calculate the fair values of performance shares reflect the probable outcome of the performance conditions as of the applicable measuring date (or actual performance results approved by the Compensation Committee as of the applicable vesting date). Outstanding RSUs accrue dividend equivalent units in the form of additional common shares that are not distributed unless and until the underlying award vests and becomes payable. The value of dividend equivalent units are reflected in the values set forth above. |  |  |
Non-PEO NEO Average Total Compensation Amount | $ 2,936,041 | $ 3,410,910 | $ 4,157,260 | $ 5,002,344
Non-PEO NEO Average Compensation Actually Paid Amount | $ 1,135,182 | 2,951,647 | 3,471,045 | 3,960,432
Adjustment to Non-PEO NEO Compensation Footnote | To calculate CAP for our PEOs and Non-PEO NEOs in the table above, the following amounts were deducted and added to the values as reported in the SCT:
​ | Fiscal Year | ​ | ​ | Average SCT Non-PEO NEOs | ​ | ​ | Pension Deducted from SCT | ​ | ​ | Pension Service Value Added to SCT | ​ | ​ | Stock Awards Deducted from SCT | ​ | ​ | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End | ​ | ​ | Change in Fair Value of Unvested Equity Awards that Vested During the Year | ​ | ​ | Change in Fair Value of Unvested Equity Grants | ​ | ​ | Average CAP Non-PEO NEOs | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​ | ​ | e | ​ | ​ | f | ​ | ​ | g | ​ | ​ | h | ​ | ​ | i | ​
​ | 1 (2023) | ​ | ​ | ​ | ​ | 2,936,041 | ​ | ​ | ​ | ​ | ​ | -478,864 | ​ | ​ | ​ | ​ | ​ | 84,416 | ​ | ​ | ​ | ​ | ​ | -1,451,776 | ​ | ​ | ​ | ​ | ​ | 827,273 | ​ | ​ | ​ | ​ | ​ | -102,269 | ​ | ​ | ​ | ​ | ​ | -679,639 | ​ | ​ | ​ | ​ | ​ | 1,135,182 | ​ | ​
​ | 2 (2022) | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | -69,941 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | -2,054,232 | ​ | ​ | ​ | ​ | ​ | 2,031,140 | ​ | ​ | ​ | ​ | ​ | -419,104 | ​ | ​ | ​ | ​ | ​ | -109,907 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​
​ | 3 (2021) | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | -1,205,814 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | -1,408,117 | ​ | ​ | ​ | ​ | ​ | 1,510,350 | ​ | ​ | ​ | ​ | ​ | -238,540 | ​ | ​ | ​ | ​ | ​ | 125,057 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​
​ | 4 (2020) | ​ | ​ | ​ | ​ | 5,002,344 | ​ | ​ | ​ | ​ | ​ | -1,929,875 | ​ | ​ | ​ | ​ | ​ | 257,161 | ​ | ​ | ​ | ​ | ​ | -1,494,123 | ​ | ​ | ​ | ​ | ​ | 1,499,609 | ​ | ​ | ​ | ​ | ​ | 422,302 | ​ | ​ | ​ | ​ | ​ | 203,013 | ​ | ​ | ​ | ​ | ​ | 3,960,432 | ​ | ​
​ (a) The total pension benefit adjustments for each applicable year include the aggregate of two components: (i) the actuarially determined service cost for services rendered by the PEO and non-PEO NEOs during the applicable year (the “service cost”); and (ii) the entire cost of benefits granted in a plan amendment (or initiation) during the applicable year that are attributed by the benefit formula to services rendered in periods prior to the plan amendment or initiation (the “prior service cost”), in each case, calculated in accordance with U.S. GAAP ​
​ | Fiscal Year | ​ | ​ | Service Cost | ​ | ​ | Prior Service Cost | ​ | ​ | Total Pension Benefit Adjustments | ​
​ | a | ​ | ​ | b | ​ | ​ | c | ​ | ​ | d | ​
​ | PEO#1 (2023) | ​ | ​ | ​ | ​ | 689,828 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 689,828 | ​ | ​
​ | PEO#1 (2022) | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​
​ | PEO#1 (2021) | ​ | ​ | ​ | ​ | 336,850 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 336,850 | ​ | ​
​ | PEO#2 (2021) | ​ | ​ | ​ | ​ | 845,242 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 845,242 | ​ | ​
​ | PEO#2 (2020) | ​ | ​ | ​ | ​ | 111,326 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 111,326 | ​ | ​
​ | Average Non-PEO NEO (2023) | ​ | ​ | ​ | ​ | 84,416 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 84,416 | ​ | ​
​ | Average Non-PEO NEO (2022) | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​
​ | Average Non-PEO NEO (2021) | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​
​ | Average Non-PEO NEO (2020) | ​ | ​ | ​ | ​ | 257,161 | ​ | ​ | ​ | ​ | ​ | 0 | ​ | ​ | ​ | ​ | ​ | 257,161 | ​ | ​
​ (b) For the periods covered, no individual equity awards were granted and vested in the same year, except as it relates to the 2023 awards to Mr. Schweiger, who retired on July 1, 2023, respectively, and no dividends or other earnings were paid on outstanding and unvested awards. The valuation assumptions used to calculate the fair values of performance shares reflect the probable outcome of the performance conditions as of the applicable measuring date (or actual performance results approved by the Compensation Committee as of the applicable vesting date). Outstanding RSUs accrue dividend equivalent units in the form of additional common shares that are not distributed unless and until the underlying award vests and becomes payable. The value of dividend equivalent units are reflected in the values set forth above. |  |  |
Compensation Actually Paid vs. Total Shareholder Return | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. TSR ​ As presented in graph below, the Company delivered positive TSR for the first three years of the four-year period, leading the EEI Index for 2020 but behind for the 2021 – 2023 periods. TSR is half of the measurement used for the Performance Share payout of our Long-Term Incentive Program. Four-year average EPS Growth is cross-referenced with the actual four-year TSR percentile to determine the actual performance share payout as a percentage of target. Based on 2023 performance, CAP for both the PEO and the Non-PEO NEO’s (average) dropped due to both the below target payout of the annual incentive plan and lower stock prices after grant. |  |  |
Compensation Actually Paid vs. Net Income | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. Net Income While the Company does not use net income as a performance measure in its overall executive compensation program and therefore would not necessarily expect a meaningful relationship to exist between the measures, the graph below does illustrate that with a reduction in net income in 2023, CAP similarly dropped for the NEO and Non-PEO NEOs in 2023. ​ |  |  |
Compensation Actually Paid vs. Company Selected Measure | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. Non-GAAP EPS ​ The graph below illustrates the trend in CAP over the four years to our non-GAAP EPS. This illustrates that over the four-year period, non-GAAP EPS consistently increased, however, compensation fluctuated due to the impact of the below target payout of our annual incentive plan in 2023 and the lower stock prices throughout 2023. |  |  |
Total Shareholder Return Vs Peer Group | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. TSR ​ As presented in graph below, the Company delivered positive TSR for the first three years of the four-year period, leading the EEI Index for 2020 but behind for the 2021 – 2023 periods. TSR is half of the measurement used for the Performance Share payout of our Long-Term Incentive Program. Four-year average EPS Growth is cross-referenced with the actual four-year TSR percentile to determine the actual performance share payout as a percentage of target. Based on 2023 performance, CAP for both the PEO and the Non-PEO NEO’s (average) dropped due to both the below target payout of the annual incentive plan and lower stock prices after grant. |  |  |
Tabular List, Table |
​ | ​ | ​ | ​ | Most Important Performance Measures | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Earnings Per Share | ​ | ​
​ | ​ | ​ | ​ | Dividend Growth | ​ | ​
​ | ​ | ​ | ​ | Total Shareholder Return | ​ | ​
​ | ​ | ​ | ​ | Reliability | ​ | ​
​ | ​ | ​ | ​ | Restoration | ​ | ​
​ | ​ | ​ | ​ | Strategic Growth Initiatives | ​ | ​
Total Shareholder Return Amount | $ 82 | 107 | 113 | 105
Peer Group Total Shareholder Return Amount | 107 | 117 | 116 | 99
Net Income (Loss) | $ (442.2) | $ 1,404.9 | $ 1,220.5 | $ 1,205.2
Company Selected Measure Amount | 4.34 | 4.09 | 3.86 | 3.64
PEO Name | Joseph R. Nolan, Jr. |  |  |
Measure:: 1 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Earnings Per Share |  |  |
Non-GAAP Measure Description | (4) Set forth in Footnote 1 on page 33 and Exhibit A to the CD&A is a full description of how non-GAAP EPS was determined. The Company believes non-GAAP EPS is a very important measure to investors and which is indicative of Company performance; accordingly, it represents a significant factor in determining both annual and long term executive compensation, though as noted the Compensation Committee determined that due to the disappointing performance in our 2023 stock price, the Committee scored the EPS goal at zero out of 200 within the EPS based portion of the 2023 annual incentive program |  |  |
Measure:: 2 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Dividend Growth |  |  |
Measure:: 3 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Total Shareholder Return |  |  |
Measure:: 4 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Reliability |  |  |
Measure:: 5 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Restoration |  |  |
Measure:: 6 |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Name | Strategic Growth Initiatives |  |  |
Joseph Nolan [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
PEO Total Compensation Amount | $ 18,885,577 | $ 12,959,134 | $ 6,467,078 |
PEO Actually Paid Compensation Amount | 7,429,247 | 10,858,066 | 5,120,192 |
Joseph Nolan [Member] | Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (7,832,472) | (2,143,443) | (1,705,782) |
Joseph Nolan [Member] | Total Pension Adjustments [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 689,828 | 429,463 | 336,850 |
Joseph Nolan [Member] | Equity Awards Value In Summary Compensation Table (Grant Date Value) |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (8,018,396) | (6,825,923) | (1,441,650) |
Joseph Nolan [Member] | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End ​[Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 6,023,948 | 6,749,195 | 1,546,319 |
Joseph Nolan [Member] | Year-end Fair Value Of Awards Granted In The Current Fiscal Year [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (182,331) | (235,802) | (201,335) |
Joseph Nolan [Member] | Change In Fair Value Of Outstanding And Unvested Awards Granted In Prior Fiscal Years [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (2,136,906) | (74,557) | 118,712 |
Joseph Nolan [Member] | Pension Adjustments Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 689,828 | 429,463 | 336,850 |
Joseph Nolan [Member] | Pension Adjustments Prior Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 0 | 0 | 0 |
James Judge [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
PEO Total Compensation Amount |  |  | 10,220,941 | $ 14,575,276
PEO Actually Paid Compensation Amount |  |  | 10,910,465 | 14,062,876
James Judge [Member] | Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 0 | (3,742,215)
James Judge [Member] | Total Pension Adjustments [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 845,242 | 111,326
James Judge [Member] | Equity Awards Value In Summary Compensation Table (Grant Date Value) |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | (6,786,337) | (6,682,612)
James Judge [Member] | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End ​[Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 7,279,051 | 6,707,152
James Judge [Member] | Year-end Fair Value Of Awards Granted In The Current Fiscal Year [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | (1,266,246) | 2,047,648
James Judge [Member] | Change In Fair Value Of Outstanding And Unvested Awards Granted In Prior Fiscal Years [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 617,814 | 1,046,301
James Judge [Member] | Pension Adjustments Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 845,242 | 111,326
James Judge [Member] | Pension Adjustments Prior Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount |  |  | 0 | 0
Non-PEO NEO | Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (478,864) | (69,941) | (1,205,814) | (1,929,875)
Non-PEO NEO | Total Pension Adjustments [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 84,416 | 162,780 | 530,849 | 257,161
Non-PEO NEO | Equity Awards Value In Summary Compensation Table (Grant Date Value) |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (1,451,776) | (2,054,232) | (1,408,117) | (1,494,123)
Non-PEO NEO | Fair Value of Equity Awards Granted during the Year and Remained Unvested at Year-End ​[Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 827,273 | 2,031,140 | 1,510,350 | 1,499,609
Non-PEO NEO | Year-end Fair Value Of Awards Granted In The Current Fiscal Year [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (102,269) | (419,104) | (238,540) | 422,302
Non-PEO NEO | Change In Fair Value Of Outstanding And Unvested Awards Granted In Prior Fiscal Years [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | (679,639) | (109,907) | 125,057 | 203,013
Non-PEO NEO | Pension Adjustments Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | 84,416 | 162,780 | 530,849 | 257,161
Non-PEO NEO | Pension Adjustments Prior Service Cost [Member] |  |  |  |
Pay vs Performance Disclosure |  |  |  |
Adjustment to Compensation, Amount | $ 0 | $ 0 | $ 0 | $ 0

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