DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
CL&P | Level 2 | Current Derivative Assets:
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	$ 0.0	$ 14.2
Netting	0.0	(0.3)
Net Amount Recorded as a Derivative	0.0	13.9
CL&P | Level 2 | Current Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.1)	(71.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(0.1)	(71.1)
NSTAR Electric | Level 3 | Current Derivative Assets:
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	91.0	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	91.0	0.0
NSTAR Electric | Level 3 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(753.1)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (753.1)	$ 0.0