INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	$ 19.8	$ 103.6
Federal and State Charitable Contribution	0.0	2.2
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	168.1	148.9
Federal and State Charitable Contribution	9.9	9.6
Operating Loss Carryforwards	65.5	80.7
CL&P | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
CL&P | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	122.3	107.0
Federal and State Charitable Contribution	0.0	0.0
Operating Loss Carryforwards	0.0	0.0
NSTAR Electric | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
NSTAR Electric | 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
PSNH | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
PSNH | 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