COMMITMENTS AND CONTINGENCIES - Schedule of Estimated Future Annual Costs of Long term Contractual Agreement (Details) $ in Millions	Dec. 31, 2019 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 998.8
2021	941.6
2022	946.5
2023	897.5
2024	881.9
Thereafter	4,785.2
Total	9,451.5
Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	614.6
2021	594.3
2022	651.7
2023	629.1
2024	624.8
Thereafter	3,590.6
Total	6,705.1
Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	73.5
2021	69.0
2022	75.1
2023	81.6
2024	75.9
Thereafter	71.5
Total	446.6
Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	22.4
2021	23.0
2022	16.9
2023	20.1
2024	15.5
Thereafter	29.8
Total	127.7
Natural Gas Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	266.2
2021	255.3
2022	202.8
2023	166.7
2024	165.7
Thereafter	1,093.3
Total	2,150.0
Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020	22.1
2021	0.0
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Total	22.1
CL&P
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	558.2
2021	547.9
2022	602.7
2023	614.3
2024	605.8
Thereafter	2,875.9
Total	5,804.8
CL&P | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	457.3
2021	459.6
2022	514.4
2023	516.2
2024	517.4
Thereafter	2,790.6
Total	5,255.5
CL&P | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	69.8
2021	65.3
2022	71.4
2023	78.0
2024	72.9
Thereafter	55.5
Total	412.9
CL&P | Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	22.4
2021	23.0
2022	16.9
2023	20.1
2024	15.5
Thereafter	29.8
Total	127.7
CL&P | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020	8.7
2021	0.0
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Total	8.7
NSTAR Electric
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	107.3
2021	94.3
2022	94.5
2023	69.6
2024	66.8
Thereafter	434.1
Total	866.6
NSTAR Electric | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	95.5
2021	91.2
2022	91.4
2023	66.6
2024	63.8
Thereafter	418.1
Total	826.6
NSTAR Electric | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	3.1
2021	3.1
2022	3.1
2023	3.0
2024	3.0
Thereafter	16.0
Total	31.3
NSTAR Electric | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020	8.7
2021	0.0
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Total	8.7
PSNH
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	67.1
2021	44.1
2022	46.5
2023	46.9
2024	43.6
Thereafter	381.9
Total	630.1
PSNH | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	61.8
2021	43.5
2022	45.9
2023	46.3
2024	43.6
Thereafter	381.9
Total	623.0
PSNH | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	0.6
2021	0.6
2022	0.6
2023	0.6
2024	0.0
Thereafter	0.0
Total	2.4
PSNH | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2020	4.7
2021	0.0
2022	0.0
2023	0.0
2024	0.0
Thereafter	0.0
Total	$ 4.7