COMMITMENTS AND CONTINGENCIES - Nuclear Fuel Litigation (Details) - USD ($) $ in Millions	Mar. 25, 2016	Sep. 30, 2016
CYAPC [Member]
Loss Contingencies [Line Items]
Damages awarded in litigation	$ 32.6
YAEC [Member]
Loss Contingencies [Line Items]
Damages awarded in litigation	19.6
MYAPC [Member]
Loss Contingencies [Line Items]
Damages awarded in litigation	24.6
Expected proceeds from Yankee Companies to be refunded to utility company		$ 57.0
Yankee Companies [Member]
Loss Contingencies [Line Items]
Damages awarded in litigation	76.8
Damages sought in litigation	$ 77.9
Eversource [Member] | Yankee Companies [Member]
Loss Contingencies [Line Items]
Expected proceeds from Yankee Companies to be refunded to utility company		26.0
The Connecticut Light And Power Company [Member] | Yankee Companies [Member]
Loss Contingencies [Line Items]
Expected proceeds from Yankee Companies to be refunded to utility company		13.6
NSTAR Electric Company [Member] | Yankee Companies [Member]
Loss Contingencies [Line Items]
Expected proceeds from Yankee Companies to be refunded to utility company		5.0
Public Service Company Of New Hampshire [Member] | Yankee Companies [Member]
Loss Contingencies [Line Items]
Expected proceeds from Yankee Companies to be refunded to utility company		3.9
Western Massachusetts Electric Company [Member] | Yankee Companies [Member]
Loss Contingencies [Line Items]
Expected proceeds from Yankee Companies to be refunded to utility company		$ 3.6