INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net Operating Loss	$ 1,079.4	$ 877.7
Tax Credit	397.4	404.1
Charitable contribution	1.1	0.0
Internal Revenue Service (IRS) | Capital Loss Carryforward
Operating Loss Carryforwards [Line Items]
Tax Credit	1,044.6	2,700.0
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net Operating Loss	1,447.8	214.1
Tax Credit	236.7	238.3
Charitable contribution	13.2	15.2
State and Local Jurisdiction | Capital Loss Carryforward
Operating Loss Carryforwards [Line Items]
Tax Credit	1,044.6	2,700.0
CL&P | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net Operating Loss	0.0	0.0
Tax Credit	0.3	0.0
Charitable contribution	0.0	0.0
CL&P | Internal Revenue Service (IRS) | Capital Loss Carryforward
Operating Loss Carryforwards [Line Items]
Tax Credit	0.0	0.0
CL&P | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net Operating Loss	0.0	0.0
Tax Credit	153.9	157.7
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
NSTAR Electric | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net Operating Loss	0.0	0.0
Tax Credit	0.0	0.0
Charitable contribution	0.0	0.0
NSTAR Electric | 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
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
PSNH | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net Operating Loss	0.0	7.1
Tax Credit	0.0	0.0
Charitable contribution	0.0	0.0
PSNH | 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
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