COMMITMENTS AND CONTINGENCIES - Harbor Civil Action (Details) $ in Millions		3 Months Ended
	Jul. 15, 2016 MW	Jun. 30, 2017 USD ($)
Loss Contingencies [Line Items]
Amount of capacity required for installation of distribution cable (in volts) | MW	0.115
NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Charge relating to assets previously recognized as capital		$ 4.9
Base rate credit for new cable		$ 17.5
Period of provision of base rate credit		7 years