COMMITMENTS AND CONTINGENCIES - Schedule of Estimated Future Annual Costs of Long term Contractual Agreement (Details) $ in Millions	Dec. 31, 2017 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	$ 599.0
2019	578.2
2020	542.8
2021	497.2
2022	459.5
Thereafter	2,869.4
Total	5,546.1
Supply and Stranded Cost
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	81.7
2019	69.3
2020	74.6
2021	68.8
2022	63.7
Thereafter	144.3
Total	502.4
Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	242.9
2019	242.5
2020	241.7
2021	232.2
2022	224.5
Thereafter	1,665.7
Total	2,849.5
Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	26.1
2019	24.2
2020	34.0
2021	32.3
2022	23.4
Thereafter	53.3
Total	193.3
Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	225.5
2019	219.2
2020	169.3
2021	148.7
2022	131.4
Thereafter	989.6
Total	1,883.7
Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2018	22.8
2019	23.0
2020	23.2
2021	15.2
2022	16.5
Thereafter	16.5
Total	117.2
The Connecticut Light And Power Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	177.9
2019	175.4
2020	198.2
2021	187.8
2022	175.6
Thereafter	836.9
Total	1,751.8
The Connecticut Light And Power Company | Supply and Stranded Cost
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	58.7
2019	56.7
2020	69.5
2021	63.7
2022	59.1
Thereafter	121.6
Total	429.3
The Connecticut Light And Power Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	84.1
2019	85.4
2020	85.5
2021	85.8
2022	86.6
Thereafter	655.5
Total	1,082.9
The Connecticut Light And Power Company | Peaker CfDs
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	26.1
2019	24.2
2020	34.0
2021	32.3
2022	23.4
Thereafter	53.3
Total	193.3
The Connecticut Light And Power Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2018	9.0
2019	9.1
2020	9.2
2021	6.0
2022	6.5
Thereafter	6.5
Total	46.3
NSTAR Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	110.6
2019	108.8
2020	104.8
2021	97.3
2022	98.0
Thereafter	517.9
Total	1,037.4
NSTAR Electric Company | Supply and Stranded Cost
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	5.5
2019	5.5
2020	3.1
2021	3.1
2022	3.1
Thereafter	22.0
Total	42.3
NSTAR Electric Company | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	96.1
2019	94.3
2020	92.6
2021	88.2
2022	88.4
Thereafter	489.4
Total	949.0
NSTAR Electric Company | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2018	9.0
2019	9.0
2020	9.1
2021	6.0
2022	6.5
Thereafter	6.5
Total	46.1
Public Service Company Of New Hampshire
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	85.0
2019	74.8
2020	70.5
2021	63.4
2022	54.5
Thereafter	525.0
Total	873.2
Public Service Company Of New Hampshire | Supply and Stranded Cost
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	17.5
2019	7.1
2020	2.0
2021	2.0
2022	1.5
Thereafter	0.7
Total	30.8
Public Service Company Of New Hampshire | Renewable Energy
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2018	62.7
2019	62.8
2020	63.6
2021	58.2
2022	49.5
Thereafter	520.8
Total	817.6
Public Service Company Of New Hampshire | Transmission Support Commitments
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2018	4.8
2019	4.9
2020	4.9
2021	3.2
2022	3.5
Thereafter	3.5
Total	$ 24.8