COMMITMENTS AND CONTINGENCIES - Schedule of Estimated Future Annual Costs of Long term Contractual Agreement (Details) $ in Millions	Dec. 31, 2018 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	$ 609.2
2020	607.9
2021	528.7
2022	494.1
2023	461.7
Thereafter	2,905.9
Total	5,607.5
Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	68.3
2020	73.5
2021	69.1
2022	72.9
2023	74.1
Thereafter	142.9
Total	500.8
Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	262.4
2020	261.0
2021	238.9
2022	240.5
2023	217.2
Thereafter	1,662.0
Total	2,882.0
Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	11.9
2020	22.6
2021	21.9
2022	15.3
2023	17.5
Thereafter	43.5
Total	132.7
Natural Gas Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	243.8
2020	227.7
2021	183.6
2022	149.2
2023	135.1
Thereafter	1,039.7
Total	1,979.1
Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2019	22.8
2020	23.1
2021	15.2
2022	16.2
2023	17.8
Thereafter	17.8
Total	112.9
The Connecticut Light And Power Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	179.9
2020	205.2
2021	197.2
2022	195.7
2023	200.4
Thereafter	959.7
Total	1,938.1
The Connecticut Light And Power Company | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	57.0
2020	69.7
2021	65.3
2022	69.1
2023	70.4
Thereafter	123.9
Total	455.4
The Connecticut Light And Power Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	102.0
2020	103.8
2021	104.0
2022	104.9
2023	105.5
Thereafter	785.3
Total	1,305.5
The Connecticut Light And Power Company | Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	11.9
2020	22.6
2021	21.9
2022	15.3
2023	17.5
Thereafter	43.5
Total	132.7
The Connecticut Light And Power Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2019	9.0
2020	9.1
2021	6.0
2022	6.4
2023	7.0
Thereafter	7.0
Total	44.5
NSTAR Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	109.2
2020	105.3
2021	97.7
2022	98.2
2023	73.9
Thereafter	461.1
Total	945.4
NSTAR Electric Company | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	5.5
2020	3.1
2021	3.1
2022	3.1
2023	3.0
Thereafter	19.0
Total	36.8
NSTAR Electric Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	94.7
2020	93.1
2021	88.6
2022	88.8
2023	63.9
Thereafter	435.1
Total	864.2
NSTAR Electric Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2019	9.0
2020	9.1
2021	6.0
2022	6.3
2023	7.0
Thereafter	7.0
Total	44.4
Public Service Company Of New Hampshire
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	76.3
2020	69.7
2021	50.2
2022	51.0
2023	52.3
Thereafter	445.4
Total	744.9
Public Service Company Of New Hampshire | Purchased Power and Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	5.8
2020	0.7
2021	0.7
2022	0.7
2023	0.7
Thereafter	0.0
Total	8.6
Public Service Company Of New Hampshire | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2019	65.7
2020	64.1
2021	46.3
2022	46.8
2023	47.8
Thereafter	441.6
Total	712.3
Public Service Company Of New Hampshire | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2019	4.8
2020	4.9
2021	3.2
2022	3.5
2023	3.8
Thereafter	3.8
Total	$ 24.0