COMMITMENTS AND CONTINGENCIES - Spent Nuclear Fuel Obligations Narrative (Details) - USD ($) $ in Millions		12 Months Ended
	Mar. 25, 2016	Dec. 31, 2016
CYAPC [Member]
Guarantor Obligations [Line Items]
Net of damages awarded	$ 32.6
YAEC [Member]
Guarantor Obligations [Line Items]
Net of damages awarded	19.6
MYAPC [Member]
Guarantor Obligations [Line Items]
Net of damages awarded	24.6	$ 14.4
Proceeds distributed from spent nuclear fuel trust		56.5
Yankee Companies [Member]
Guarantor Obligations [Line Items]
Net of damages awarded	76.8
Damages sought	$ 77.9
Eversource [Member] | Yankee Companies [Member]
Guarantor Obligations [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company		26.1
The Connecticut Light And Power Company [Member] | Yankee Companies [Member]
Guarantor Obligations [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company		13.6
NSTAR Electric Company [Member] | Yankee Companies [Member]
Guarantor Obligations [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company		5.0
Public Service Company Of New Hampshire [Member] | Yankee Companies [Member]
Guarantor Obligations [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company		3.9
Western Massachusetts Electric Company [Member] | Yankee Companies [Member]
Guarantor Obligations [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company		$ 3.6