DERIVATIVE INSTRUMENTS - Variations Using Significant unobservable Inputs (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (423.3)	$ (380.9)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(11.4)	(130.7)
Settlements	71.2	88.3
Transfer out of Level 3	1.2
Ending balance	(362.3)	(423.3)
The Connecticut Light And Power Company
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(420.5)	(380.8)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(9.5)	(122.7)
Settlements	67.7	83.0
Transfer out of Level 3	0.0
Ending balance	$ (362.3)	$ (420.5)