REGULATORY ACCOUNTING - FERC ROE Complaints (Details) - FERC ROE First and Second Complaints $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	$ 39.1
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	21.4
NSTAR Electric Company
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	14.6
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers	$ 3.1