DERIVATIVE INSTRUMENTS - Variations Using Significant unobservable Inputs (Details) - Fair Value, Inputs, Level 3 - The Connecticut Light And Power Company - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (362.3)	$ (420.5)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(32.0)	(9.5)
Settlements	37.8	67.7
Ending balance	$ (356.5)	$ (362.3)