REGULATORY ACCOUNTING - FERC ROE Complaints (Details) - USD ($)					12 Months Ended
	May 01, 2020	May 01, 2019	May 01, 2018	Nov. 30, 2017	Dec. 31, 2017
Regulatory Assets [Line Items]
Annual distribution rate increase				$ 37,000,000
Revenue requirement reduction				56,000,000
Decrease resulting from revenue requirement reduction				$ 19,000,000
Percentage of authorized regulatory ROE				10.00%	9.25%
Period of stay-out requirement				5 years
Period of recovery approved in previously expensed merger-related costs				10 years
Pre-tax benefit for approved regulatory deferral costs and expensed costs that have been disallowed					$ 44,100,000
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Annual distribution rate increase				$ 0.53
NSTAR Electric Company
Regulatory Assets [Line Items]
Pre-tax benefit for approved regulatory deferral costs and expensed costs that have been disallowed					14,100,000
FERC ROE First and Second Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers					39,100,000
FERC ROE First and Second Complaints | The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers					21,400,000
FERC ROE First and Second Complaints | NSTAR Electric Company
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers					14,600,000
FERC ROE First and Second Complaints | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers					$ 3,100,000
Scenario, Forecast
Regulatory Assets [Line Items]
Annual distribution rate increase	$ 24,700,000	$ 32,700,000	$ 97,100,000