INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	$ 0.0	$ 19.8
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	186.6	168.1
Federal and State Charitable Contribution	10.2	9.9
Operating Loss Carryforwards	183.4	65.5
The Connecticut Light and Power Company | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
The Connecticut Light and Power Company | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	133.4	122.3
Federal and State Charitable Contribution	0.0	0.0
Operating Loss Carryforwards	0.0	0.0
NSTAR Electric Company | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
NSTAR Electric Company | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
Operating Loss Carryforwards	0.0	0.0
Public Service Company of New Hampshire | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Public Service Company of New Hampshire | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
Operating Loss Carryforwards	$ 0.0	$ 0.0