COMMITMENTS AND CONTINGENCIES - Schedule of Estimated Future Annual Costs of Long term Contractual Agreement (Details) $ in Millions	Dec. 31, 2016 USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	$ 667.8
2018	557.1
2019	498.4
2020	483.2
2021	433.9
Thereafter	2,331.8
Total	4,972.2
Supply and Stranded Cost [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	115.8
2018	81.6
2019	69.4
2020	74.2
2021	58.4
Thereafter	189.8
Total	589.2
Renewable Energy [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	275.4
2018	242.6
2019	240.9
2020	238.8
2021	218.9
Thereafter	1,864.1
Total	3,080.7
Peaker CfDs [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	42.3
2018	21.5
2019	21.7
2020	31.1
2021	27.6
Thereafter	54.2
Total	198.4
Natural Gas Procurement [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	197.0
2018	185.5
2019	142.3
2020	115.0
2021	104.9
Thereafter	190.2
Total	934.9
Coal, Wood and Other [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	15.5
2018	3.9
2019	1.9
2020	1.9
2021	1.9
Thereafter	11.3
Total	36.4
Transmission Support Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2017	21.8
2018	22.0
2019	22.2
2020	22.2
2021	22.2
Thereafter	22.2
Total	132.6
The Connecticut Light And Power Company [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	222.2
2018	169.3
2019	167.4
2020	189.0
2021	170.0
Thereafter	909.7
Total	1,827.6
The Connecticut Light And Power Company [Member] | Supply and Stranded Cost [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	93.4
2018	58.7
2019	56.6
2020	68.8
2021	53.0
Thereafter	162.3
Total	492.8
The Connecticut Light And Power Company [Member] | Renewable Energy [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	77.9
2018	80.4
2019	80.3
2020	80.3
2021	80.6
Thereafter	684.4
Total	1,083.9
The Connecticut Light And Power Company [Member] | Peaker CfDs [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	42.3
2018	21.5
2019	21.7
2020	31.1
2021	27.6
Thereafter	54.2
Total	198.4
The Connecticut Light And Power Company [Member] | Transmission Support Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2017	8.6
2018	8.7
2019	8.8
2020	8.8
2021	8.8
Thereafter	8.8
Total	52.5
NSTAR Electric Company [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	128.4
2018	92.7
2019	90.9
2020	86.6
2021	82.1
Thereafter	448.6
Total	929.3
NSTAR Electric Company [Member] | Supply and Stranded Cost [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	4.8
2018	5.5
2019	5.5
2020	3.1
2021	3.1
Thereafter	25.0
Total	47.0
NSTAR Electric Company [Member] | Renewable Energy [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	116.8
2018	80.4
2019	78.5
2020	76.6
2021	72.1
Thereafter	416.7
Total	841.1
NSTAR Electric Company [Member] | Transmission Support Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2017	6.8
2018	6.8
2019	6.9
2020	6.9
2021	6.9
Thereafter	6.9
Total	41.2
Public Service Company Of New Hampshire [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	102.9
2018	92.1
2019	80.2
2020	74.8
2021	59.0
Thereafter	620.4
Total	1,029.4
Public Service Company Of New Hampshire [Member] | Supply and Stranded Cost [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	17.6
2018	17.4
2019	7.3
2020	2.3
2021	2.3
Thereafter	2.5
Total	49.4
Public Service Company Of New Hampshire [Member] | Renewable Energy [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	65.2
2018	66.1
2019	66.3
2020	65.9
2021	50.1
Thereafter	601.9
Total	915.5
Public Service Company Of New Hampshire [Member] | Coal, Wood and Other [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	15.5
2018	3.9
2019	1.9
2020	1.9
2021	1.9
Thereafter	11.3
Total	36.4
Public Service Company Of New Hampshire [Member] | Transmission Support Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2017	4.6
2018	4.7
2019	4.7
2020	4.7
2021	4.7
Thereafter	4.7
Total	28.1
Western Massachusetts Electric Company [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	17.3
2018	17.5
2019	17.6
2020	17.8
2021	17.9
Thereafter	162.9
Total	251.0
Western Massachusetts Electric Company [Member] | Renewable Energy [Member]
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	15.5
2018	15.7
2019	15.8
2020	16.0
2021	16.1
Thereafter	161.1
Total	240.2
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2017	1.8
2018	1.8
2019	1.8
2020	1.8
2021	1.8
Thereafter	1.8
Total	$ 10.8