COMMITMENTS AND CONTINGENCIES - Long-Term Contractual Arrangement Narrative (Details) MWh in Millions, $ in Millions					12 Months Ended
	Oct. 31, 2020	Nov. 22, 2019 megawatt	Mar. 15, 2019 megawatt	Dec. 28, 2018 MWh	Dec. 31, 2020 USD ($) contract project MW	Dec. 31, 2019 MW
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts					$ 11,170.6
Number of capacity contracts for differences between CL&P and UI | contract					3
Amount of capacity contracts for differences between CL&P and UI (in MW) | MW					675	676
Number of generation projects between CL&P and United Illuminated Company | project					2
Number of demand response projects between CL&P and United Illuminated Company | project					1
Percentage of costs and benefits borne by United Illuminated Company					20.00%
Contract term					20 years
Construction in Progress
Long-term Purchase Commitment [Line Items]
Total construction contract cost					$ 6,700.0
Minimum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract					150.0
Maximum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract					415.0
Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts					$ 286.4
Percentage of costs and benefits borne by United Illuminated Company					20.00%
Period of payment to generation facility owner					30 years
The Connecticut Light and Power Company
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts					$ 6,012.8
The Connecticut Light and Power Company | Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts					$ 286.4
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				30 years
Millstone Nuclear Power Station and Seabrook Nuclear Power Plant
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts					$ 3,600.0
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%
Nameplate capacity (in MW) | megawatt			2,112
Milestone Nuclear Power Station | The Connecticut Light and Power Company
Long-term Purchase Commitment [Line Items]
Contract term			10 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			10 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%
Nameplate capacity (in MW) | megawatt		1,250
Seabrook Nuclear Power Plant | The Connecticut Light and Power Company
Long-term Purchase Commitment [Line Items]
Contract term		8 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		8 years		8 years