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INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
State and Local Jurisdiction    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss $ 214.1 $ 243.4
Tax Credit 238.3 228.5
State Charitable Contribution 15.2 7.9
State and Local Jurisdiction | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 2,700.0 0.0
Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 877.7 0.0
Tax Credit 404.1 0.0
Internal Revenue Service (IRS) | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 2,700.0 0.0
CL&P | State and Local Jurisdiction    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 0.0 0.0
Tax Credit 157.7 157.5
State Charitable Contribution 0.0 0.0
CL&P | State and Local Jurisdiction | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 0.0 0.0
CL&P | Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 0.0 0.0
Tax Credit 0.0 0.0
CL&P | Internal Revenue Service (IRS) | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 0.0 0.0
NSTAR Electric | State and Local Jurisdiction    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 0.0 0.0
Tax Credit 0.0 0.0
State Charitable Contribution 0.0 0.0
NSTAR Electric | State and Local Jurisdiction | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 0.0 0.0
NSTAR Electric | Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 0.0 0.0
Tax Credit 0.0 0.0
NSTAR Electric | Internal Revenue Service (IRS) | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 0.0 0.0
PSNH | State and Local Jurisdiction    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 0.0 0.0
Tax Credit 0.0 0.0
State Charitable Contribution 0.0 0.0
PSNH | State and Local Jurisdiction | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit 0.0 0.0
PSNH | Internal Revenue Service (IRS)    
Operating Loss Carryforwards [Line Items]    
Net Operating Loss 7.1 0.0
Tax Credit 0.0 0.0
PSNH | Internal Revenue Service (IRS) | Capital Loss Carryforward    
Operating Loss Carryforwards [Line Items]    
Tax Credit $ 0.0 $ 0.0