COMMITMENTS AND CONTINGENCIES - Schedule of Estimated Future Annual Costs of Long term Contractual Agreement (Details) $ in Millions	Dec. 31, 2020 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	$ 1,328.8
2022	1,218.6
2023	1,096.1
2024	1,052.3
2025	1,023.0
Thereafter	5,451.8
Total	11,170.6
Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	650.0
2022	716.8
2023	652.3
2024	648.8
2025	651.5
Thereafter	3,490.7
Total	6,810.1
Natural Gas Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	558.9
2022	374.1
2023	292.0
2024	263.1
2025	260.2
Thereafter	1,825.8
Total	3,574.1
Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	69.4
2022	75.5
2023	86.8
2024	79.6
2025	60.3
Thereafter	13.2
Total	384.8
Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	30.0
2022	34.2
2023	46.9
2024	41.6
2025	31.3
Thereafter	102.4
Total	286.4
Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2021	20.5
2022	18.0
2023	18.1
2024	19.2
2025	19.7
Thereafter	19.7
Total	115.2
The Connecticut Light and Power Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	588.3
2022	661.5
2023	684.6
2024	674.9
2025	647.7
Thereafter	2,755.8
Total	6,012.8
The Connecticut Light and Power Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	484.4
2022	548.3
2023	547.2
2024	549.1
2025	551.2
Thereafter	2,645.5
Total	5,325.7
The Connecticut Light and Power Company | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	65.8
2022	71.9
2023	83.3
2024	76.6
2025	57.4
Thereafter	0.1
Total	355.1
The Connecticut Light and Power Company | Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	30.0
2022	34.2
2023	46.9
2024	41.6
2025	31.3
Thereafter	102.4
Total	286.4
The Connecticut Light and Power Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2021	8.1
2022	7.1
2023	7.2
2024	7.6
2025	7.8
Thereafter	7.8
Total	45.6
NSTAR Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	109.2
2022	110.2
2023	85.5
2024	83.4
2025	83.7
Thereafter	542.9
Total	1,014.9
NSTAR Electric Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	98.0
2022	100.0
2023	75.4
2024	72.9
2025	73.1
Thereafter	522.1
Total	941.5
NSTAR Electric Company | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	3.1
2022	3.1
2023	3.0
2024	3.0
2025	2.9
Thereafter	13.1
Total	28.2
NSTAR Electric Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2021	8.1
2022	7.1
2023	7.1
2024	7.5
2025	7.7
Thereafter	7.7
Total	45.2
Public Service Company of New Hampshire
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	72.4
2022	72.8
2023	34.0
2024	30.9
2025	31.4
Thereafter	327.3
Total	568.8
Public Service Company of New Hampshire | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	67.6
2022	68.5
2023	29.7
2024	26.8
2025	27.2
Thereafter	323.1
Total	542.9
Public Service Company of New Hampshire | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2021	0.5
2022	0.5
2023	0.5
2024	0.0
2025	0.0
Thereafter	0.0
Total	1.5
Public Service Company of New Hampshire | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2021	4.3
2022	3.8
2023	3.8
2024	4.1
2025	4.2
Thereafter	4.2
Total	$ 24.4