COMMITMENTS AND CONTINGENCIES - Summary of Exposure to Guaruantees and Indemnifications (Details) $ in Millions	Jun. 30, 2019 USD ($)
Eversource Investment LLC - Declining Balance Guarantee for North East Offshore
Loss Contingencies [Line Items]
Maximum exposure	$ 113.9
Various - Surety Bonds
Loss Contingencies [Line Items]
Maximum exposure	34.0
Rocky River Realty Company and Eversource Service - Lease Payments for Real Estate
Loss Contingencies [Line Items]
Maximum exposure	7.1
Bay State Wind LLC - Real Estate Purchase
Loss Contingencies [Line Items]
Maximum exposure	$ 2.5