COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2022 USD ($) site	Dec. 31, 2021 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	62	61
Reserve | $	$ 121.3	$ 115.4
CL&P
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 14.3	$ 13.9
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	11	11
Reserve | $	$ 3.6	$ 3.3
PSNH
Site Contingency [Line Items]
Number of Sites | site	9	9
Reserve | $	$ 6.3	$ 6.3