INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	$ 103.6	$ 197.3
Federal and State Charitable Contribution	2.2	18.7
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	148.9	139.0
Federal and State Charitable Contribution	9.6	31.4
Operating Loss Carryforwards	80.7	82.8
The Connecticut Light And Power Company | 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
The Connecticut Light And Power Company | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	107.0	94.5
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
Federal and State Charitable Contribution	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
Federal and State Charitable Contribution	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