INCOME TAXES - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	$ 2,900,000,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Valuation allowance for tax credit and operating loss carryforward	14,400,000	$ 4,500,000
Minimum
Operating Loss Carryforwards [Line Items]
Decrease in unrecognized tax benefits	0
Maximum
Operating Loss Carryforwards [Line Items]
Decrease in unrecognized tax benefits	2,200,000
Eversource | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	9,900,000	1,300,000
The Connecticut Light And Power Company
Operating Loss Carryforwards [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	1,000,000,000
The Connecticut Light And Power Company | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	1,800,000	$ 1,300,000
NSTAR Electric Company
Operating Loss Carryforwards [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	1,100,000,000
Public Service Company Of New Hampshire
Operating Loss Carryforwards [Line Items]
Deferred Income tax benefit related to tax cuts and jobs act	$ 400,000,000