REGULATORY ACCOUNTING - FERC ROE Complaints (Details) - FERC ROE First and Second Complaints $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	$ 39.1
CL&P
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	21.4
NSTAR Electric
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	14.6
PSNH
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	$ 3.1