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, 2023 USD ($) MW contract
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 12,956.3
Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 103.7
Percentage of costs and benefits borne by United Illuminated Company				20.00%
Period of payment to generation facility owner				30 years
CL&P
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 5,550.4
CL&P | Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 103.7
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
NSTAR Electric
Long-term Purchase Commitment [Line Items]
Contract term (in years)				20 years
Estimated cost of energy purchase contracts				$ 4,012.7
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
Millstone Nuclear Power Station and Seabrook Nuclear Power Plant
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 2,400.0
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
Vineyard Wind Project | NSTAR Electric
Long-term Purchase Commitment [Line Items]
Contract term (in years)				20 years
Nameplate capacity (in MW) | MW				800