REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands			3 Months Ended			9 Months Ended
	Jun. 27, 2019	May 01, 2019	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Apr. 17, 2019	Apr. 16, 2019	Mar. 26, 2019	Dec. 31, 2018	Nov. 16, 2018
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved			$ 144,300			$ 144,300					$ 122,900
Regulatory assets			4,830,800			4,830,800					5,145,900
Other income, net			26,968		$ 16,718	103,818	$ 100,656
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved			44,700			44,700					42,100
Regulatory assets			1,577,800			1,577,800					1,630,700
Other income, net			4,831		7,098	11,564	20,722
CL&P | Storm Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved									$ 145,500			$ 153,000
Recovery period		6 years
Regulatory assets		$ 141,000						$ 141,000
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved			17,800			17,800					12,200
Regulatory assets			870,400			870,400					929,500
Other income, net			4,634		16,095	14,640	24,253
PSNH | Storm Costs
Regulatory Assets [Line Items]
Recovery period	5 years
Regulatory assets	$ 68,500									$ 38,100
PSNH | Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Other income, net				$ 5,200	8,700	5,200	8,700
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved			61,500			61,500					49,300
Regulatory assets			1,360,300			1,360,300					$ 1,438,200
Other income, net			$ 10,735		$ 13,697	$ 32,479	$ 40,567