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COMMITMENTS AND CONTINGENCIES - Long-Term Contractual Arrangement Narrative (Details) MWh in Millions, $ in Millions |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Nov. 22, 2019 | Mar. 15, 2019 | Dec. 28, 2018 MWh | Dec. 31, 2024 USD ($) contract MW
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated cost of energy purchase contracts |  |  |  | $ 13,961.4
Peaker Contracts For Differences |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated cost of energy purchase contracts |  |  |  | $ 211.1
Percentage of costs and benefits borne by United Illuminated Company |  |  |  | 20.00%
Period of payment to generation facility owner |  |  |  | 30 years
NSTAR Electric |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  |  | 20 years
Estimated cost of energy purchase contracts |  |  |  | $ 3,780.5
NSTAR Electric | Construction in Progress |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Total construction contract cost |  |  |  | 2,600.0
NSTAR Electric | Minimum |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated costs under construction contract |  |  |  | 100.0
NSTAR Electric | Maximum |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated costs under construction contract |  |  |  | 200.0
CL&P |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated cost of energy purchase contracts |  |  |  | 7,164.6
CL&P | Peaker Contracts For Differences |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated cost of energy purchase contracts |  |  |  | $ 211.1
Number of capacity contracts | contract |  |  |  | 3
Amount of peaking capacity contracts (in MW) | MW |  |  |  | 500
Percentage of costs and benefits borne |  |  |  | 80.00%
CL&P, NSTAR Electric and PSNH | Transmission Support Commitments |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Period of payment of annual operation and maintenance expense |  |  |  | 20 years
Vineyard Wind Project | NSTAR Electric |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  |  | 20 years
Millstone Nuclear Power Station and Seabrook Nuclear Power Plant |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Estimated cost of energy purchase contracts |  |  |  | $ 2,600.0
Milestone Nuclear Power Station |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Annual purchase requirement (in MWh) | MWh |  |  | 9 |
Percent of the facility's output required to be purchased |  | 50.00% |  |
Milestone Nuclear Power Station | CL&P |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  | 10 years |
Percent of the facility's output required to be purchased |  | 40.00% |  |
Milestone Nuclear Power Station | UI |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  | 10 years |
Seabrook Nuclear Power Plant |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Percent of the facility's output required to be purchased | 18.00% |  |  |
Seabrook Nuclear Power Plant | CL&P |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  | 8 years |
Percent of the facility's output required to be purchased | 15.00% |  |  |
Seabrook Nuclear Power Plant | UI |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |
Contract term (in years) |  |  | 8 years |

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