INCOME TAXES - Open Tax Year (Details) $ in Billions	12 Months Ended
	Dec. 31, 2017 USD ($)
Income Tax Contingency [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	$ 2.9
Federal
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2017
Maximum | Connecticut
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2014
Maximum | Massachusetts
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2014
Maximum | New Hampshire
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2015
Minimum | Connecticut
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2017
Minimum | Massachusetts
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2017
Minimum | New Hampshire
Income Tax Contingency [Line Items]
Open tax years that remain subject to examination	2017
The Connecticut Light And Power Company
Income Tax Contingency [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	$ 1.0
NSTAR Electric Company
Income Tax Contingency [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	1.1
Public Service Company Of New Hampshire
Income Tax Contingency [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	$ 0.4