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Pay vs Performance Disclosure | 12 Months Ended
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Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Pay vs Performance Disclosure [Table] |  |  |
Pay vs Performance [Table Text Block] | Pay Versus Performance ​ Pursuant to Item 402(v) of Regulation S-K, we are providing the following information regarding certain measures of the relationship between executive compensation and our financial performance for each of the last three completed calendar years. In determining the Compensation Actually Paid (CAP) to our NEOs, we make various adjustments to amounts that have been previously reported in the Summary Compensation Table (SCT) in previous years, as the SEC’s valuation methods for this section differ from those required in the SCT. Note that for our NEOs other than our principal executive officer (the PEO), compensation is reported as average compensation. In the 2022 Annual Incentive Program Performance Assessment as well as the Long-Term Incentive Program sections of the CD&A, we describe all of the elements that have a direct connection to the Company’s financial performance and to the CAP of the Company’s PEO and the NEOs as a group (excluding the PEO). As the majority of total direct compensation is provided to our PEO and NEOs through long-term incentives, the Company’s stock price also has a direct relationship to the CAP. ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​ | ​ | ​ | ​ | 107 | ​ | ​ | ​ | ​ | ​ | 117 | ​ | ​ | ​ | ​ | ​ | 1,404.9 | ​ | ​ | ​ | ​ | ​ | 4.09 | ​ | ​
​ | (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 | ​ | ​
​ | (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. 2022 and 2021 — Joseph Nolan ​ b. 2020 and 2021 — James 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. 2022 — John Moreira, Werner Schweiger, Gregory Butler, Christine Carmody, James Judge, and Philip Lembo ​ b. 2021 — Philip Lembo, Werner Schweiger, Gregory Butler, and Christine Carmody ​ c. 2020 — Philip Lembo, Werner Schweiger, Gregory Butler, and Joseph Nolan ​ (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 and December 31, 2022 as compared to the Edison Electric Institute (“EEI”) Index, as disclosed in our Annual Report on Form 10-K for fiscal year ended 2022 pursusant to Item 201(e) of Regulation S-K (4) Set forth in Footnote 1 on page 36 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 ​ 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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | -2,143,443 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | -6,825,923 | ​ | ​ | ​ | ​ | ​ | 6,749,195 | ​ | ​ | ​ | ​ | ​ | -235,802 | ​ | ​ | ​ | ​ | ​ | -74,557 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​
​ | (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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | -69,941 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | -2,054,232 | ​ | ​ | ​ | ​ | ​ | 2,031,140 | ​ | ​ | ​ | ​ | ​ | -419,104 | ​ | ​ | ​ | ​ | ​ | -109,907 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​
​ | (2021) | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | -1,205,814 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | -1,408,117 | ​ | ​ | ​ | ​ | ​ | 1,510,350 | ​ | ​ | ​ | ​ | ​ | -238,540 | ​ | ​ | ​ | ​ | ​ | 125,057 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​
​ | (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 (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 (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 2022 awards to Mr. Lembo and Mr. Judge, who retired on November 30, 2022 and December 31, 2022, 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 [Text Block] | (1) The PEOs for each year are as follows: ​ a. 2022 and 2021 — Joseph Nolan ​ b. 2020 and 2021 — James 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. 2022 — John Moreira, Werner Schweiger, Gregory Butler, Christine Carmody, James Judge, and Philip Lembo ​ b. 2021 — Philip Lembo, Werner Schweiger, Gregory Butler, and Christine Carmody ​ c. 2020 — Philip Lembo, Werner Schweiger, Gregory Butler, and Joseph Nolan |  |
Peer Group Issuers, Footnote [Text Block] | (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 and December 31, 2022 as compared to the Edison Electric Institute (“EEI”) Index, as disclosed in our Annual Report on Form 10-K for fiscal year ended 2022 pursusant to Item 201(e) of Regulation S-K |  |
Adjustment To PEO Compensation, Footnote [Text Block] | 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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | -2,143,443 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | -6,825,923 | ​ | ​ | ​ | ​ | ​ | 6,749,195 | ​ | ​ | ​ | ​ | ​ | -235,802 | ​ | ​ | ​ | ​ | ​ | -74,557 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​
​ | (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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | -69,941 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | -2,054,232 | ​ | ​ | ​ | ​ | ​ | 2,031,140 | ​ | ​ | ​ | ​ | ​ | -419,104 | ​ | ​ | ​ | ​ | ​ | -109,907 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​
​ | (2021) | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | -1,205,814 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | -1,408,117 | ​ | ​ | ​ | ​ | ​ | 1,510,350 | ​ | ​ | ​ | ​ | ​ | -238,540 | ​ | ​ | ​ | ​ | ​ | 125,057 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​
​ | (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 (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 (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 2022 awards to Mr. Lembo and Mr. Judge, who retired on November 30, 2022 and December 31, 2022, 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 | $ 3,410,910 | $ 4,157,260 | $ 5,002,344
Non-PEO NEO Average Compensation Actually Paid Amount | $ 2,951,647 | 3,471,045 | 3,960,432
Adjustment to Non-PEO NEO Compensation Footnote [Text Block] | 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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 12,959,134 | ​ | ​ | ​ | ​ | ​ | -2,143,443 | ​ | ​ | ​ | ​ | ​ | 429,463 | ​ | ​ | ​ | ​ | ​ | -6,825,923 | ​ | ​ | ​ | ​ | ​ | 6,749,195 | ​ | ​ | ​ | ​ | ​ | -235,802 | ​ | ​ | ​ | ​ | ​ | -74,557 | ​ | ​ | ​ | ​ | ​ | 10,858,066 | ​ | ​
​ | (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 | ​
​ | (2022) | ​ | ​ | ​ | ​ | 3,410,910 | ​ | ​ | ​ | ​ | ​ | -69,941 | ​ | ​ | ​ | ​ | ​ | 162,780 | ​ | ​ | ​ | ​ | ​ | -2,054,232 | ​ | ​ | ​ | ​ | ​ | 2,031,140 | ​ | ​ | ​ | ​ | ​ | -419,104 | ​ | ​ | ​ | ​ | ​ | -109,907 | ​ | ​ | ​ | ​ | ​ | 2,951,647 | ​ | ​
​ | (2021) | ​ | ​ | ​ | ​ | 4,157,260 | ​ | ​ | ​ | ​ | ​ | -1,205,814 | ​ | ​ | ​ | ​ | ​ | 530,849 | ​ | ​ | ​ | ​ | ​ | -1,408,117 | ​ | ​ | ​ | ​ | ​ | 1,510,350 | ​ | ​ | ​ | ​ | ​ | -238,540 | ​ | ​ | ​ | ​ | ​ | 125,057 | ​ | ​ | ​ | ​ | ​ | 3,471,045 | ​ | ​
​ | (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 (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 (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 2022 awards to Mr. Lembo and Mr. Judge, who retired on November 30, 2022 and December 31, 2022, 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 [Text Block] | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. TSR ​ As presented in graph below, the Company delivered positive TSR over three-year period, leading the EEI Index for 2020 but behind for the 2021 and 2022 periods. TSR is half of the measurement used for the Performance Share payout of our Long-Term Incentive Program. Three-year average EPS Growth is cross-referenced with the actual three-year TSR percentile to determine the actual performance share payout as a percentage of target. Similar to the previous two graphs, compensation, however, fluctuated due to the impact of the change in executive roster over the same three year period. ​ |  |
Compensation Actually Paid vs. Net Income [Text Block] | Relationship Between CAP for the PEOs and Non-PEO NEOs (Average) vs. Net Income ​ The graph below illustrates that over the three year period, net income consistently increased. 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. ​ |  |
Compensation Actually Paid vs. Company Selected Measure [Text Block] | 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 three years to our non-GAAP EPS. This illustrates that over the three year period, EPS consistently increased. Compensation, however, fluctuated due to the impact of the change in executive roster over the same three year period. |  |
Tabular List [Table Text Block] | Most Important Company Performance Measures for Determining Executive Compensation ​ As also required by Section 953(a) of the Dodd-Frank Wall Street Reform and Consumer Protection Act and Item 402(v) of Regulation S-K, below is a list of the performance measures that are considered the most important by the Company in determining executive compensation for the 2022 performance year. The performance measures included in this table are not ranked by relative importance. These are described in the 2022 Performance Goals section.
​ | ​ | ​ | ​ | Most Important Performance Measures | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Earnings per share | ​ | ​
​ | ​ | ​ | ​ | Dividend Growth | ​ | ​
​ | ​ | ​ | ​ | Total Shareholder Return | ​ | ​
​ | ​ | ​ | ​ | Reliability | ​ | ​
​ | ​ | ​ | ​ | Restoration | ​ | ​
​ | ​ | ​ | ​ | Strategic Growth Initiatives | ​ | ​
Total Shareholder Return Amount | $ 107 | 113 | 105
Peer Group Total Shareholder Return Amount | 117 | 116 | 99
Net Income (Loss) | $ 1,404.9 | $ 1,220.5 | $ 1,205.2
Company Selected Measure Amount | 4.09 | 3.86 | 3.64
PEO Name | Joseph Nolan |  |
Measure [Axis]: 1 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Earnings per share |  |
Non-GAAP Measure Description [Text Block] | (4) Set forth in Footnote 1 on page 36 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 ​ |  |
Measure [Axis]: 2 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Dividend Growth |  |
Measure [Axis]: 3 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Total Shareholder Return |  |
Measure [Axis]: 4 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Reliability |  |
Measure [Axis]: 5 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Restoration |  |
Measure [Axis]: 6 |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Measure Name | Strategic Growth Initiatives |  |
Joseph Nolan [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
PEO Total Compensation Amount | $ 12,959,134 | $ 6,467,078 |
PEO Actually Paid Compensation Amount | 10,858,066 | 5,120,192 |
Joseph Nolan [Member] | Deduct: Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (2,143,443) | (1,705,782) |
Joseph Nolan [Member] | Pension Adjustments Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 429,463 | 336,850 |
Joseph Nolan [Member] | Deduct: Equity Awards Value In Summary Compensation Table (Grant Date Value) [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (6,825,923) | (1,441,650) |
Joseph Nolan [Member] | Fair Value of Equity Awards Granted during the Year and Remained Unvested [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 6,749,195 | 1,546,319 |
Joseph Nolan [Member] | Change in Fair Value of Unvested Equity Awards that Vested During the Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (235,802) | (201,335) |
Joseph Nolan [Member] | Change in Fair Value of Unvested Equity Grants [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (74,557) | 118,712 |
Joseph Nolan [Member] | Pension Adjustments Prior Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 0 | 0 |
Joseph Nolan [Member] | Pension Benefit Adjustment [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 429,463 | 336,850 |
James Judge. Mr. Nolan [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
PEO Total Compensation Amount |  | 10,220,941 | $ 14,575,276
PEO Actually Paid Compensation Amount |  | 10,910,465 | 14,062,876
James Judge. Mr. Nolan [Member] | Deduct: Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 0 | (3,742,215)
James Judge. Mr. Nolan [Member] | Pension Adjustments Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 845,242 | 111,326
James Judge. Mr. Nolan [Member] | Deduct: Equity Awards Value In Summary Compensation Table (Grant Date Value) [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | (6,786,337) | (6,682,612)
James Judge. Mr. Nolan [Member] | Fair Value of Equity Awards Granted during the Year and Remained Unvested [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 7,279,051 | 6,707,152
James Judge. Mr. Nolan [Member] | Change in Fair Value of Unvested Equity Awards that Vested During the Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | (1,266,246) | 2,047,648
James Judge. Mr. Nolan [Member] | Change in Fair Value of Unvested Equity Grants [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 617,814 | 1,046,301
James Judge. Mr. Nolan [Member] | Pension Adjustments Prior Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 0 | 0
James Judge. Mr. Nolan [Member] | Pension Benefit Adjustment [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount |  | 845,242 | 111,326
Non-PEO NEO [Member] | Deduct: Pension Adjustments Pension Value In Summary Compensation Table [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (69,941) | (1,205,814) | (1,929,875)
Non-PEO NEO [Member] | Pension Adjustments Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 162,780 | 530,849 | 257,161
Non-PEO NEO [Member] | Deduct: Equity Awards Value In Summary Compensation Table (Grant Date Value) [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (2,054,232) | (1,408,117) | (1,494,123)
Non-PEO NEO [Member] | Fair Value of Equity Awards Granted during the Year and Remained Unvested [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 2,031,140 | 1,510,350 | 1,499,609
Non-PEO NEO [Member] | Change in Fair Value of Unvested Equity Awards that Vested During the Year [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (419,104) | (238,540) | 422,302
Non-PEO NEO [Member] | Change in Fair Value of Unvested Equity Grants [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | (109,907) | 125,057 | 203,013
Non-PEO NEO [Member] | Pension Adjustments Prior Service Cost [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | 0 | 0 | 0
Non-PEO NEO [Member] | Pension Benefit Adjustment [Member] |  |  |
Pay vs Performance Disclosure [Table] |  |  |
Adjustment to Compensation Amount | $ 162,780 | $ 530,849 | $ 257,161

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