INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Operating Loss Carryforwards [Line Items]
Tax liability related to the deferral of employer payroll tax liability provision of the CARES Act			$ 39.0
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Valuation allowance for tax credit and operating loss carryforward	$ 48.3	$ 43.0
Eversource | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	10.3	18.5
The Connecticut Light and Power Company | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 8.8	$ 14.2