COMMITMENTS AND CONTINGENCIES - Harbor Civil Action (Details) $ in Millions		3 Months Ended
	Jul. 15, 2016 MW	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Amount of capacity required for installation of distribution cable (in volts) | MW	0.115
NSTAR Electric Company
Loss Contingencies [Line Items]
Charge relating to assets previously recognized as capital		$ 4.9
Base rate credit for new cable		17.5
Loss contingency accrual		$ 2.1	$ 11.1