INCOME TAXES - Narrative (Details) - State and Local Jurisdiction - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Valuation allowance for tax credit and operating loss carryforward	$ 19.5	$ 14.4
Eversource
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	5.2	9.9
The Connecticut Light And Power Company
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 4.4	$ 1.8