INCOME TAXES - Narrative (Details) - State and Local Jurisdiction - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Valuation allowance for tax credit and operating loss carryforward	$ 43.0	$ 19.5
Eversource
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	18.5	5.2
CL&P
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 14.2	$ 4.4