LEASES - Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 55.2	$ 49.9
Operating Lease Liabilities - Current Portion	9.5	8.6
Operating Lease Liabilities - Long-Term	45.7	41.3
Total Operating Lease Liabilities	55.2	49.9
Finance Leases:
Finance Lease Right-of-use-Assets, Net	60.5	8.2
Finance Lease Liabilities - Current Portion	5.0	2.4
Finance Lease Liabilities - Long-Term	57.6	8.1
Total Finance Lease Liabilities	$ 62.6	$ 10.5
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentNet	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentNet
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
The Connecticut Light and Power Company
Operating Leases:
Operating Lease Right-of-use-Assets, Net	$ 0.3	$ 0.7
Operating Lease Liabilities - Current Portion	0.2	0.5
Operating Lease Liabilities - Long-Term	0.1	0.2
Total Operating Lease Liabilities	0.3	0.7
Finance Leases:
Finance Lease Right-of-use-Assets, Net	0.7	1.9
Finance Lease Liabilities - Current Portion	1.4	1.6
Finance Lease Liabilities - Long-Term	0.0	1.4
Total Finance Lease Liabilities	1.4	3.0
NSTAR Electric Company
Operating Leases:
Operating Lease Right-of-use-Assets, Net	23.6	24.2
Operating Lease Liabilities - Current Portion	0.7	0.7
Operating Lease Liabilities - Long-Term	22.9	23.5
Total Operating Lease Liabilities	23.6	24.2
Finance Leases:
Finance Lease Right-of-use-Assets, Net	3.5	3.3
Finance Lease Liabilities - Current Portion	0.0	0.0
Finance Lease Liabilities - Long-Term	4.8	4.4
Total Finance Lease Liabilities	4.8	4.4
Public Service Company of New Hampshire
Operating Leases:
Operating Lease Right-of-use-Assets, Net	0.3	0.4
Operating Lease Liabilities - Current Portion	0.0	0.1
Operating Lease Liabilities - Long-Term	0.3	0.3
Total Operating Lease Liabilities	0.3	0.4
Finance Leases:
Finance Lease Right-of-use-Assets, Net	0.8	0.9
Finance Lease Liabilities - Current Portion	0.1	0.1
Finance Lease Liabilities - Long-Term	0.7	0.8
Total Finance Lease Liabilities	$ 0.8	$ 0.9