REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 17, 2018	May 08, 2018
Regulatory Assets [Line Items]
Deferred storm restoration costs		$ 5,145,900	$ 5,239,300
Other Income, Net		128,366	107,913	$ 64,505
Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs		266,000
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Deferred storm restoration costs		929,500	940,800
Amount Of Recovery Approved In Storm Restoration Costs					$ 49,000
Other Income, Net		27,672	9,805	7,563
Public Service Company Of New Hampshire | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs		24,000
Other Income, Net	$ 8,700
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Deferred storm restoration costs		1,630,700	1,645,200
Other Income, Net		22,663	22,991	14,238
The Connecticut Light And Power Company | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs		148,000
NSTAR Electric Company
Regulatory Assets [Line Items]
Deferred storm restoration costs		1,438,200	1,524,500
Other Income, Net		53,066	$ 34,100	$ 21,263
NSTAR Electric Company | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Deferred storm restoration costs		$ 94,000
Rate Reduction Bonds | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued						$ 635,700