INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	$ 197.3	$ 8.6
Federal and State Charitable Contribution	18.7	27.8
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	139.0	111.1
Federal and State Charitable Contribution	31.4	36.5
Operating Loss Carryforwards	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	94.5	80.5
Federal and State Charitable Contribution	0.0	0.0
Operating Loss Carryforwards	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
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