DERIVATIVE INSTRUMENTS - Variations Using Significant unobservable Inputs (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (380.9)	$ (415.4)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(130.7)	(52.1)
Settlements	88.3	86.6
Ending balance	(423.3)	(380.9)
The Connecticut Light And Power Company [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(380.8)	(410.9)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(122.7)	(51.3)
Settlements	83.0	81.4
Ending balance	(420.5)	(380.8)
NSTAR Electric Company [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(0.1)	(4.5)
Net Realized/Unrealized Losses Included in Regulatory Assets and Liabilities	(8.0)	(0.8)
Settlements	5.3	5.2
Ending balance	$ (2.8)	$ (0.1)