ASSET RETIREMENT OBLIGATIONS - Narrative (Details) $ in Millions		12 Months Ended
	Jul. 15, 2016 kV	Dec. 31, 2019 USD ($) kV	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Asset Retirement Obligation [Line Items]
Asset retirement obligations		$ 489.5	$ 466.2	$ 419.1
Amount of capacity required for installation of distribution cable (in kV) | kV	115	115
Eversource and NSTAR Electric | Other Current Liabilities
Schedule of Asset Retirement Obligation [Line Items]
Asset retirement obligations		$ 1.0
CYAPC and YAEC
Schedule of Asset Retirement Obligation [Line Items]
Asset retirement obligations		$ 337.7	$ 339.9