COMMITMENTS AND CONTINGENCIES - Long-Term Contractual Arrangement Narrative (Details) MWh in Millions, $ in Millions				12 Months Ended
	Nov. 22, 2019 megawatt	Mar. 15, 2019 megawatt	Dec. 28, 2018 MWh	Dec. 31, 2019 USD ($) contract project MW	Dec. 31, 2008 contract MW	Dec. 31, 2018 MW
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 9,451.5
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				676		787
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%
Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 127.7
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				5,804.8
CL&P | Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 127.7
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				30 years
Millstone Nuclear Power Station and Seabrook Nuclear Power Plant
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 4,100.0
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 | CL&P
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 | CL&P
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