DERIVATIVE INSTRUMENTS - Variations Using Significant unobservable Inputs (Details) - Level 3 - CL&P - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (356.5)	$ (362.3)
Net Realized/Unrealized Losses Included in Regulatory Assets	(15.0)	(32.0)
Settlements	42.3	37.8
Ending balance	$ (329.2)	$ (356.5)