REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	May 06, 2021
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 260,600	$ 196,900
Regulatory costs	6,475,200	6,569,900
Regulatory Liabilities	612,931	389,430
Unfavorable Regulatory Action
Regulatory Assets [Line Items]
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards			$ 28,400
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	944,200	765,600
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs related to non-tracked uncollectible expense	34,500	15,800
Net incremental deferred costs	41,300	24,000
Tropical Storm Isaias | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	251,000
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	111,200	84,100
Regulatory costs	2,307,100	2,211,800
Regulatory Liabilities	311,269	137,166
CL&P | Loss from Catastrophes
Regulatory Assets [Line Items]
Deferred pre-staging and storm restoration costs	195,000	344,000
CL&P | Unfavorable Regulatory Action
Regulatory Assets [Line Items]
Regulatory Liabilities	75,000
CL&P | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	674,200	515,100
CL&P | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	15,900	4,700
CL&P | Tropical Storm Isaias | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	234,000
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	85,100	69,800
Regulatory costs	1,636,100	1,703,900
Regulatory Liabilities	205,771	164,761
NSTAR Electric | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	198,100	186,400
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	10,900	11,900
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3,200	4,300
Regulatory costs	927,700	989,100
Regulatory Liabilities	96,817	58,756
PSNH | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	$ 71,900	$ 64,100