ASSET RETIREMENT OBLIGATIONS - Narrative (Details) $ in Millions	Jul. 15, 2016 kV	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Asset Retirement Obligation [Line Items]
Asset retirement obligations		$ 499.7	$ 489.5	$ 466.2
Amount of capacity required for installation of distribution cable (in kV) | kV	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		$ 330.3	$ 337.7