COMMITMENTS AND CONTINGENCIES - Guarantees and Indemnifications (Details) $ in Millions	Sep. 30, 2021 USD ($)
North East Offshore LLC
Guarantor Obligations [Line Items]
Ownership interest	50.00%
Sunrise Wind LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 15.4
Ownership interest	50.00%
South Fork Wind, LLC
Guarantor Obligations [Line Items]
Ownership interest	50.00%
Eversource Investment LLC | North East Offshore LLC
Guarantor Obligations [Line Items]
Ownership interest	50.00%
Construction-related purchase agreements with third-party contractors | North East Offshore LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 1,093.5
Construction-related purchase agreements with third-party contractors | Sunrise Wind LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	386.4
Construction-related purchase agreements with third-party contractors | South Fork Wind, LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	40.6
Funding and indemnification obligations of North East Offshore LLC | Eversource Investment LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	0.0
Funding and indemnification obligations of North East Offshore LLC | Eversource Investment LLC | North East Offshore LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	910.0
OREC capacity production | Sunrise Wind LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	2.2
Transmission interconnection | South Fork Wind, LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	1.2
Real estate purchase | Bay State Wind LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	2.5
Surety bonds | Various
Guarantor Obligations [Line Items]
Maximum exposure (up to)	67.5
Lease payments for real estate | Rocky River Realty Company and Eversource Service
Guarantor Obligations [Line Items]
Maximum exposure (up to)	4.0
Performance guarantee | North East Offshore LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 1,300.0
Percent of obligations guaranteed	50.00%
Performance guarantee | South Fork Wind, LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 14.8
Payment guarantee | North East Offshore LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 62.5
Percent of obligations guaranteed	50.00%
Payment guarantee | South Fork Wind, LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 25.8
Construction-Related Purchase Guarantee | Sunrise Wind LLC
Guarantor Obligations [Line Items]
Maximum exposure (up to)	$ 420.6